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
hg init rep
cd rep
mkdir dir
touch foo dir/bar
hg -v addremove
hg -v commit -m "add 1" -d "1000000 0"
cd dir/
touch ../foo_2 bar_2
hg -v addremove
hg -v commit -m "add 2" -d "1000000 0"