bitbake: hob: ensure error message text is properly escaped

bitbake: hob: ensure error message text is properly escaped

Our poor implementation of markup escaping was causing invalid
markup, leading to the error dialog being blank. Use the glib markup
escaping function provided by PyGTK+ to do this properly and avoid the
blank error dialogs.

Partial fix for [YOCTO #2983].

(Bitbake rev: 563ea5233a5ab1629c51e802d04280692f96c596)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>