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.
Is there an error message when trying to diff non-existing files?
found: No such file or directory
not: No such file or directory
Is there an error message when trying to add non-existing files?
found: No such file or directory
not: No such file or directory