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=hg
"$HG" init
mkdir b
echo 'Bouncy' >b/bouncy
echo 'tricycle' >b/vehicle
"$HG" add b/bouncy
"$HG" add b/vehicle
"$HG" commit -m 'Adding bouncy'
echo 'bouncy' >>b/bouncy
"$HG" commit -m 'Making it bouncier'
"$HG" update -C 0
echo 'stationary' >>b/vehicle
"$HG" commit -m 'Clarifying the vehicle.'
"$HG" update -C 1
chmod a-w b/vehicle
"$HG" update -m 2 2>&1 | sed 's|^\(.*[ ]\).*/\([^/]*/[^/]*/[^/]*\)$|\1\2|g'