Auto merge of #1955 - carols10cents:even-more-manifest-path-consistency, r=alexcrichton

Hiii :) This builds on #1953, so if yinz like this better, I'll close that one.

This PR makes all commands that take `--manifest-path` (except locate-project, see below) behave in a consistent way with regards to invalid paths, and hopefully gives better error messages.

This DOES break some instances where commands were "working", ie, completing their command successfully, with some path that they decided was right, not necessarily exactly the path that was passed in though. I'm not sure how this impacts whatever backwards compatibility guarantees cargo has. Given that those cases weren't using a valid path to an existing Cargo.toml, hopefully no one is relying on that behavior?

So locate-project... I'm not sure what to do about that one, exactly... If we define `--manifest-path` as a path to a Cargo.toml, then uhhhh we've located the project, it's right where you told us it is ;) So I think we should change it to be `--search-path` or similar? I'm too sleepy to take care of that tonight, so I just left it out of the manifest path tests.