[Checkers] errorKey on BaseTypeVisitor

[Telmo]

> 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.

Well, yes. I meant to ask whether I should just keep this change, or 
change the javari tests to use type.incompatible (which would make some 
bugs pass by unnoticed), or do something else. Flags to determine which 
key to use also seem like a framework hack, though. I just reverted 
BaseTypeVisitor to use type.incompatible; let me know of any other 
suggestions on how to do this, even though this has low priority.

-Telmo