mercurial.el: ignore errors on hg-after-save-hook()
Don't signal an error even if running hg command fails. Otherwise,
succeeding hooks won't be called.
#!/bin/sh
mkdir t
cd t
hg init
echo a > a
hg add a
hg commit -m "test" -d "1000000 0"
hg verify
hg parents
hg status
hg rollback
hg verify
hg parents
hg status