Report better error for phantom inner class references

[pwrightcertinia]

Currently if you try and make use of a phantom inner class (see https://github.com/ChuckJonas/wtfapex#phantom-inner-class-type-equivalency) then you just get the standard Unknown Type error. This can be very confusing if you don't know about this issue. Ideally we would give better information based on the outer type existing.

I think this should remain an error, we don't want to support such a horrible bug, its deeply confusing when you try read the code.