R1:3c3e00a74f77

Raise early fatal for bad rewrite with no "/"

Summary: This particular misconfiguration results in a difficult-to-debug redirect loop, so stop it early.

Test Plan: Broke my rewrite rule, verified I got yelled at.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D4591