Propagate error code if between 400 and 600 for production sites

[Perlkonig]

Fix for issue #2164.

The code is only propagated if both of the following are true, otherwise the default 500 is thrown:

• system:errors:display is 0 or -1
• The error code is between 400 and 599

The first condition is necessary because when verbosity is 1, it uses the built-in Whoops handler. The only way to override (that I can find) is to create our own PrettyPageHandler that inherits from the built-in one and that contains the propagation code. But I wanted a minimal solution for now. I am happy to take the next step if this approach is acceptable.

I tested this by throwing a new RuntimeException in Grav\Common\Twig\Twig::processSite. I also tested throwing with no code. Certainly more tests are necessary, but I don't know enough about the possible repercussions.

Let me know if you'd like me to make further changes.

[rhukster]

Merging!