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