[Checkers] errorKey on BaseTypeVisitor

[Matt Papi]

> 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.

This change breaks the NonNull and Interned test suites (they were 
refactored to use "type.incompatible" a couple weeks ago). It's really a 
good idea to run the checkers' test suites before committing a change in 
checkers.{source,basetype,types,util}.* to make sure that other checkers 
don't break.

If you need the old error keys for testing purposes, we could (for 
example) add a flag that causes them to be used in place of 
"type.incompatible". However, I think we should leave 
"type.incompatible" there, since it allows checkers to emit nicer, 
javac-like errors by default.

(Also, it looks like the "type.incompatible" change is still just a 
hack. Are there plans to go back and do it in a cleaner way?)

- Matt