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

mkdir a
cd a
hg init
echo foo > b
hg add b
hg ci -m "b" -d "1000000 0"

chmod -w .hg

cd ..

hg clone a b

chmod +w a/.hg # let test clean up

cd b
hg verify