Improve "arc land" error message to mention SVN alternatives

Summary: Fixes T3813. This error message isn't very helpful in SVN; be more helpful.

Test Plan: Ran arc land in a Subversion repository, got a reasonable error message.

Reviewers: btrahan, asherkin

Reviewed By: asherkin

Subscribers: aran

Maniphest Tasks: T3813

Differential Revision: https://secure.phabricator.com/D7275