Make java artifact caching less sloppy.

Also make creating the processors info file happen even
when we pulled from the artifact cache.
Fix invalidation bugs.

1) Delete the hash-file before building a target, so that if
   the build aborts and leaves the output dir in a bad state
   and then we revert the change, we force a rebuild.

2) Ensure hashes are computed consistently.
Make missing dep errors actual errors.

Remove some commented-out code that wouldn't even work
any more if it were uncommented.

Auditors: markcc
Revert "Make missing dep errors actual errors."

This reverts commit b5f47ae1cc4b5889b8a50bfd767e7848cca275f4.
Fix scala artifact merging bug.
optionally autokill nailguns