(stable) Don't fatal when encountering ...

Summary:
Fixes T12796. In D17647, the parser became more strict, but this remarkup rule doesn't deal with it gracefully.

Instead, detect when the parse failed and bail out.

Test Plan:

• Put [[ http://good.com#u:p@evil.com/ | broken ]] into a Remarkup document without backticks.
• Before patch: fatal ("rejecting ambiguous URI").
• After patch: link doesn't work (which is correct), but page does.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12796

Differential Revision: https://secure.phabricator.com/D18076