oe/utils: log exceptions in ThreadedWorker functions

If the function a ThreadedWorker is executing raises an exception, don't
use print() as that mostly disappears.  Instead, output it to the logger.

This is done using bb.mainlogger.debug directly instead of bb.debug() as
this allows us to pass the exception instance directly, which is then
incorporated into the log stream.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 2f1ea25c)
Signed-off-by: Steve Sakoman <steve@sakoman.com>