bitbake: build.py: add unhandled exception information to the logger when executing...

bitbake: build.py: add unhandled exception information to the logger when executing python tasks

Previously this information was simply discarded, which in some cases
resulted in generic, unhelpful failures.

With this change the user would see what the exception is, but
without ugly tracebacks or bulky, irrelevant information from the log file.

(Bitbake rev: 8da9242702fbef8cf3156b95a1076802e0f653c7)

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>