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.
unknown
0acdaf898367 tip
0acdaf8983679e0aac16e811534eb49d7ee1f2b4 first
tip 1:8a3ca90d111dc784e6575d373105be12570e8776
first 0:0acdaf8983679e0aac16e811534eb49d7ee1f2b4
8a3ca90d111d tip
M a
8a3ca90d111d+ tip
0acdaf898367+ first
0acdaf8983679e0aac16e811534eb49d7ee1f2b4+ first
M a
8216907a933d tip
8216907a933d+8a3ca90d111d+ tip
M .hgtags
.hgtags:2: ignoring invalid tag
.hgtags:4: ignoring invalid tag
localtags:1: ignoring invalid tag
tip 4:fd868a874787a7b5af31e1675666ce691c803035
first 0:0acdaf8983679e0aac16e811534eb49d7ee1f2b4
changeset: 4:fd868a874787
.hgtags:2: ignoring invalid tag
.hgtags:4: ignoring invalid tag
localtags:1: ignoring invalid tag
tag: tip
user: test
date: Mon Jan 12 13:46:40 1970 +0000
summary: tags