When a linter raises a message at a nonexistent line, don't fatal during rendering

Summary:
See PHI1782. If a linter raises a message at a line which does not exist in the file, render a confused warning rather than fataling.

This is a long-existing issue which was exacerbated by D21044.

Test Plan: Modified a linter to raise issues at line 99999. Before change: fatal in console rendering. After change: reasonable rendering.

Differential Revision: https://secure.phabricator.com/D21357