dispatch: factor out command failure handling into a function Moving the warning generation to a function allows for wrapping and alternative error handling.

pypy: fix overeager pattern matching on mpatchError Pypy have difference in error reporting.

mpatch: unify mpatchError (issue5182) The pure version was mpatch was throwing struct.error or ValueError for errors, whereas the C version was throwing an "mpatch.mpatchError". Introducing an mpatch.mpatchError into pure and using it consistently is fairly easy, but the actual form for it is mercurial.mpatch.mpatchError, so with this commit, we change the C implementation to match the naming convention too.