Make ./bin/diviner generate more fault tolerant

Summary: Allow ./bin/diviner generate to continue even if there is an exception throw processing an atom. This allows Diviner documentation to be generated for PHP source code that cannot be parsed with XHPAST.

Test Plan: Ran ./bin/diviner generate on a PHP repository which previously throw an XHPASTSyntaxErrorException.

Reviewers: btrahan, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D10803