Promote warnings to exceptions in ext/pcre

[kocsismate]

No description provided.

[kocsismate]

Bonus question: should we treat regex compilation errors as an exception?

[nikic]

I don't think we'll want to do these changes. As we don't have a dedicated function to validate a regular expression, doing a preg_match() on dummy input and checking for false is the way to do so now, and this PR would break it -- while an invalid regex itself would still return false, invalid modifiers (that are also part of the regex) would throw, etc.