In Append2Simg and Append in verity_utils.py, we catch and rethrow
exceptions as BuildVerityImageError. However, this suppresses the
traceback from the original exception which usually indicates the actual
cause. We can better handle this with the raise statement in Python 3,
which is however unavailable in Python 2.
This CL logs the exception before rethrowing to retain the useful bits.
Test: Inject an error to append2simg. `m -j systemimage` with
aosp_marlin-userdebug. Check the output.
Change-Id: I0c2f57d6023fa1038256b85fa98d57ad0244a70d
46901fb8ca