Bugfix: mbstring polyfills must not raise value errors in PHP 7

[derrabus]

trafficstars

Fixes #499.

This PR changes all new mbstring polyfills on PHP 7 so that they trigger an oldschool PHP warning instead of raising a ValueError. The reason for that is that the ValueError class might not be available on PHP 7 and the behavior is more consitent with the other polyfills of the mbstring extension.

The somewhat weird side-effect is that we now polyfill a behavior that was never implemented in PHP.

[nicolas-grekas]

Let's also address #506 here?