Provide more info for bsdiff/imgdiff exceptions.

When bsdiff/imgdiff fails, dump the output along with the src/tgt
filenames and block ranges. Also, exit the script gracefully instead
of raising an exception about iterating over 'NoneType'.

Bug: 31381530
Test: Package generates successfully for angler, and error outputs
      correctly with error injection.

Change-Id: I06a2dfe545fbdff7043de05fee34b378453a9291