(stable) Fix an exception in the hyperlink remarkup rule for unparseable URIs

Summary:
Ref T12526. URIs in the form http://x.y/#http://x.y/# can fail to parse. Catch these cases and ignore them, rather than throwing.

See also similar changes earlier, in D18149 and D18076.

Test Plan:

• Added failing test cases, made them pass.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12526

Differential Revision: https://secure.phabricator.com/D18666