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 > t1
hg add t1
hg commit -m "1" -d "1000000 0"
cd ..
hg clone a b
cd a
echo foo > t2
hg add t2
hg commit -m "2" -d "1000000 0"
cd ../b
echo foo > t3
hg add t3
hg commit -m "3" -d "1000000 0"
hg push ../a
hg pull ../a
hg push ../a
hg up -m
hg commit -m "4" -d "1000000 0"
hg push ../a