fix backtrace printed when cannot get lock.
change lock error handling code so exceptions have useful info and
exception handling in one place.
add test case for when cannot get lock.
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1 files, 1 changesets, 1 total revisions
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
verify failed
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
1 files, 1 changesets, 1 total revisions
commit failed