bitbake: utils: Improve better_exec traceback handling

bitbake: utils: Improve better_exec traceback handling

The current bitbake tracebacks are hard to read/confusing and sometimes
incomplete. This patch attempts to do better by:

* Moving the note about the exact exception to the end to make things
  read in sequence
* Merged the initial stack trace to become part of the code dump
* Added handling for "/xxxx" file paths since we can load these files
  and include the data as part of the trace
* Dropped the ERROR: prefix to every line, allowing the error messages to
  be spacially accosicated in the UIs
* Moved the "From:" line to the top of each code block and ensured its present
  consistently

With the complexity now in this funciton, I've added try/except wrapping around
it to ensure we catch exceptions in the exception handler too.

Example before:

"""
ERROR: Error executing a python function in /media/build1/poky/meta/recipes-core/eglibc/eglibc-initial_2.17.bb:
TypeError: 'filter' object is not subscriptable

ERROR: The stack trace of python calls that resulted in this exception/failure was:
ERROR:   File "do_populate_lic", line 13, in <module>
ERROR:
ERROR:   File "do_populate_lic", line 6, in do_populate_lic
ERROR:
ERROR:   File "license.bbclass", line 99, in find_license_files
ERROR:
ERROR:   File "/media/build1/poky/meta/lib/oe/license.py", line 38, in visit_string
ERROR:     if pos > 0 and license_pattern.match(elements[pos-1]):
ERROR:
ERROR: The code that was being executed was:
ERROR:      0009:    destdir = os.path.join(d.getVar('LICSSTATEDIR', True), d.getVar('PN', True))
ERROR:      0010:    copy_license_files(lic_files_paths, destdir)
ERROR:      0011:
ERROR:      0012:
ERROR:  *** 0013:do_populate_lic(d)
ERROR:      0014:
ERROR: [From file: 'do_populate_lic', lineno: 13, function: <module>]
ERROR:      0002:def do_populate_lic(d):
ERROR:      0003:    """
ERROR:      0004:    Populate LICENSE_DIRECTORY with licenses.
ERROR:      0005:    """
ERROR:  *** 0006:    lic_files_paths = find_license_files(d)
ERROR:      0007:
ERROR:      0008:    # The base directory we wrangle licenses to
ERROR:      0009:    destdir = os.path.join(d.getVar('LICSSTATEDIR', True), d.getVar('PN', True))
ERROR:      0010:    copy_license_files(lic_files_paths, destdir)
ERROR: [From file: 'do_populate_lic', lineno: 6, function: do_populate_lic]
ERROR:      0095:        lic_files_paths.append((os.path.basename(path), srclicfile))
ERROR:      0096:
ERROR:      0097:    v = FindVisitor()
ERROR:      0098:    try:
ERROR:  *** 0099:        v.visit_string(license_types)
ERROR:      0100:    except oe.license.InvalidLicense as exc:
ERROR:      0101:        bb.fatal('%s: %s' % (d.getVar('PF', True), exc))
ERROR:      0102:    except SyntaxError:
ERROR:      0103:        bb.warn("%s: Failed to parse it's LICENSE field." % (d.getVar('PF', True)))
ERROR: [From file: 'license.bbclass', lineno: 99, function: find_license_files]
ERROR: Function failed: do_populate_lic
ERROR: Logfile of failure stored in: /media/build1/poky/build/tmp/work/i586-poky-linux/eglibc-initial/2.17-r3/temp/log.do_populate_lic.17442
"""

Example after:

"""
ERROR: Error executing a python function in /media/build1/poky/meta/recipes-core/eglibc/eglibc-initial_2.17.bb:

The stack trace of python calls that resulted in this exception/failure was:
File: 'do_populate_lic', lineno: 13, function: <module>
     0009:    destdir = os.path.join(d.getVar('LICSSTATEDIR', True), d.getVar('PN', True))
     0010:    copy_license_files(lic_files_paths, destdir)
     0011:
     0012:
 *** 0013:do_populate_lic(d)
     0014:
File: 'do_populate_lic', lineno: 6, function: do_populate_lic
     0002:def do_populate_lic(d):
     0003:    """
     0004:    Populate LICENSE_DIRECTORY with licenses.
     0005:    """
 *** 0006:    lic_files_paths = find_license_files(d)
     0007:
     0008:    # The base directory we wrangle licenses to
     0009:    destdir = os.path.join(d.getVar('LICSSTATEDIR', True), d.getVar('PN', True))
     0010:    copy_license_files(lic_files_paths, destdir)
File: 'license.bbclass', lineno: 99, function: find_license_files
     0095:        lic_files_paths.append((os.path.basename(path), srclicfile))
     0096:
     0097:    v = FindVisitor()
     0098:    try:
 *** 0099:        v.visit_string(license_types)
     0100:    except oe.license.InvalidLicense as exc:
     0101:        bb.fatal('%s: %s' % (d.getVar('PF', True), exc))
     0102:    except SyntaxError:
     0103:        bb.warn("%s: Failed to parse it's LICENSE field." % (d.getVar('PF', True)))
File: '/media/build1/poky/meta/lib/oe/license.py', lineno: 38, function: visit_string
     0034:        new_elements = []
     0035:        elements = filter(lambda x: x.strip(), license_operator.split(licensestr))
     0036:        for pos, element in enumerate(elements):
     0037:            if license_pattern.match(element):
 *** 0038:                if pos > 0 and license_pattern.match(elements[pos-1]):
     0039:                    new_elements.append('&')
     0040:                element = '"' + element + '"'
     0041:            elif not license_operator.match(element):
     0042:                raise InvalidLicense(element)
Exception: TypeError: 'filter' object is not subscriptable

ERROR: Function failed: do_populate_lic
ERROR: Logfile of failure stored in: /media/build1/poky/build/tmp/work/i586-poky-linux/eglibc-initial/2.17-r3/temp/log.do_populate_lic.3275
ERROR: Task 9 (/media/build1/poky/meta/recipes-core/eglibc/eglibc-initial_2.17.bb, do_populate_lic) failed with exit code '1
"""

(Bitbake rev: c5de66b870406d9bd1161a9b7e2b04fe6eb065fe)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>