Verify remotes ("paths") in Mercurial during "arc land"

Summary: Ref T13546. Parse "hg paths" and validate that the remotes "arc land" plans to interact with actually exist.

Test Plan: Ran "arc land" with good and bad "--into-remote" and "--onto-remote" arguments, got sensible validation behavior.

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21342