R1:a2f909f0bd0a

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…