treemanifest: fail gracefully in prefetch if no remote is configured
If no remote path is configured raise an Abort exception, rather than returning
the path as None and crashing later on with an unfriendly exception due to
trying to use None as a string.
Test Plan: Updated test-treemanifest-prefetch.t to exercise this code.
Reviewers: singhsrb, durham, #fbhgext
Reviewed By: durham, #fbhgext
Differential Revision: https://phab.mercurial-scm.org/D1541