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.
requesting all changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
pulling from ../source2
pulling from ../source1
requesting all changes
adding changesets
adding manifests
adding file changes
added 10 changesets with 10 changes to 1 files
(run 'hg update' to get a working copy)
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
(run 'hg update' to get a working copy)
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
1 files, 11 changesets, 11 total revisions