bitbake: utils.py: Try harder to extract good traceback information by querying the...

bitbake: utils.py: Try harder to extract good traceback information by querying the datastore

Currently as soon as execution passes outside the code fragment being
executed by better_exec, we don't get any good traceback information,
just a likely obscure reference to some function name which may
or may not be identifiable.

This patch adds code to query the datastore if present, allowing a more
meaningful back trace to be displayed in many cases.

[YOCTO #2981]

(Bitbake rev: 0edf8431f9ff52581afe0d3ef525c59909af02ba)

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