Don't throw exceptions in __toString

[Daniel15]

:xhp:raw-pcdata-element::stringify() can throw an exception, which is not allowed by PHP as it can occur in a __toString call (renderChild method). This is enforced by PHP 5.5 which will throw a fatal error if you try to throw an exception in a __toString call (effectively getting rid of all the exception information 👎)

[Swahvay]

aced031