Fix hhvm error message not including the name of the function

I replaced the strpos match by a preg_match.
I look in the source of pcre extension to find error that can be triggered (https://github.com/php/php-src/blob/bf59acdea75cf13d179f10ce89d296a30f38676d/ext/pcre/php_pcre.c#L225)

If you prefer, I can only test if the error is a warning instead...