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.
pulling from ../B1
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)
bar should remain deleted.
f9b0e817f6a48de3564c6b2957687c5e7297c5a0 644 foo
pulling from ../A2
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 0 changes to 0 files (+1 heads)
(run 'hg update' to get a working copy)
bar should remain deleted.
f9b0e817f6a48de3564c6b2957687c5e7297c5a0 644 foo