Improve XHPAST handling of syntax errors

Summary: Currently, a bunch of developers are using #xhpast for writing custom linter rules. As such, we end up with a fair few XHPASTSyntaxErrorException in our PHP error logs. I think that throwing an exception is not quite correct in this case because it is somewhat expected that invalid PHP may be entered. Instead, catch the exception and show the user a helpful message.

Test Plan: This doesn't quite work yet... the stream and tree views render as blank but the exceptions still propogate to the error logs. Mostly, I'm not sure how the exception should be rendered for display.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14028