releasetools: Log the exceptions before rethrowing.

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