More robust error responses

[mattfarina]

This is a work in progress

@technosophos @sdboyer any feedback?

[sdboyer]

Not immediately obvious to me where this would be used. I don't remember all the flows exactly, but isn't this the kind of thing where you'd only encounter this if you try to explicitly use the wrong type on an existing on-disk repo?

[mattfarina]

ErrWrongVCS is an error that exists today. It happens when the type of VCS you already have cloned locally is mismatched with the remote location type. Say the local VCS is bzr and the endpoint is git.

[mattfarina]

Note, this did show up in Glide when folks migrated from Google Code (Hg) to GitHub.

[sdboyer]

Sorry, totally me just boneheadedly note scrolling far enough down in the diff to see these already existed

[mattfarina]

@technosophos and @sdboyer I made some major changes from the first commit.

1. Each VCS operation returns the same error text as the other VCS. This means error responses are consistent.
2. There are RemoteError and LocalError to distinguish a little more. Basically, was there a remote communication or not. I preferred this over a type for each method.
3. The original error and output string of a command (if there was one) is available on the error. This is useful if you want to output the underlying things or see what was really going on.

If there's any feedback please let me know.