dispatch: split global error handling out so it can be reused
We may want a similar error handling at worker.py. This patch extracts the
error handling logic to "callcatch" so it can be reused.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state