[Checkers] errorKey on BaseTypeVisitor

[Telmo]

The only change I committed outside of the javari checker was modificating 
which line was commented out and which line was active on BaseTypeVisitor;
as of now it uses the actualy provided error key, not "type.incompatible",
to test that the error messages are in fact being cased by the right 
reasons.

(Also, thanks for Mahmood for helping me set up Eclipse. Actually 
following the visitors step by step, monitoring the variables, let me 
found the bug I've been hunting by the past week, and mostly fix it.)

-Telmo