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
echo foo > a
hg add a
hg commit -m "1" -d "1000000 0"
hg verify
chmod -r .hg/data/a.d
hg verify 2>/dev/null || echo verify failed
chmod +r .hg/data/a.d
hg verify 2>/dev/null || echo verify failed
chmod -w .hg/data/a.d
echo barber > a
hg commit -m "2" -d "1000000 0" 2>/dev/null || echo commit failed