[BuildGetter] Check for failures when calling pushlog_lookup()

Don't throw uncaught exceptions, instead make sure all builds without
full info simply cannot be prepare'd
[BuildGetter] Don't lookup CompileBuild revisions in Hg

Redundant with pushlog. Also use pushlog commit for everything
[BuildGetter] Add build.get_valid() instead of throwing exceptions on prepare()
[BatchTester] Use build.get_valid() to determine a build should continue