Include captures in parsed errors

[joshaber]

Some of our error regexes include captures, but they aren't actually passed up and out to consumers. We should do that, though it'll require some refactoring of errors.

[shiftkey]

Seems legit. Probably a breaking change to the API as parseError is only returning an enum currently:

public static parseError(stderr: string): GitError | null {
  // ...
}