Convert common.RunAndWait method to raise an exception on subprocess failure.

Then refactor the code in merge_target_files.py to adapt to this semantic
change. This makes the code more consistent with existing releasetools code,
and it's easier to follow.

Test: Failure cases (verify exception), success cases (merged target generated)
Bug: 124521133
Change-Id: I56f04e360d8ff8ffcd6245359cdeb79f4565a9c4