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.
#!/bin/sh
#
# Corrupt an hg repo with two pulls.
#
# create one repo with a long history
hg init source1
cd source1
touch foo
hg add foo
for i in 1 2 3 4 5 6 7 8 9 10; do
echo $i >> foo
hg ci -m $i
done
cd ..
# create a third repo to pull both other repos into it
hg init version2
hg -R version2 pull source1 &
sleep 1
hg clone --pull -U version2 corrupted
hg -R corrupted verify
hg -R version2 verify