Merge pull request #2653 from lambdageek/dev/exception_type-1

Merge pull request #2653 from lambdageek/dev/exception_type-1

[runtime] Don't access MonoClass::exception_type directly.

Use `mono_class_get_failure ()` and `mono_class_has_failed ()`

This is mostly just cleanup refactoring at this point that will make it easier to change how exception_type is used.

Also from this exercise we've learned that most of the class handling code just cares whether the class has an error or not, not what sort of error it was.