Error handling in the renamer

[Jan Stolarek]

I'm improving my patch for injective type families and I have a question about error handling in 
the renamer. If I write this incorrect definition:

  type family F a b = r | r -> c

I get these errors:

  T6018rnfail.hs:36:1: Not in scope: type variable ‘c’

  T6018rnfail.hs:36:26:
    Unexpected type variable on the RHS of injectivity condition: ‘c’
    All variables should be bound in type family head and appear at most once in exactly the same 
order as they were bound.

I don't want to report two errors here, just the first one. This means I have to first run the 
renaming using RnTypes.rnTyVar and check if it reports errors. If it does I don't report the 
extra error. The question is how do I check if this piece of code returns errors:

        injFrom' <- rnTyVar True injFrom
        injTo'   <- mapM (rnTyVar True) injTo

I found some combinators in TcRnMonad (tryTc and friends) but I don't know how to use them. 
Ideally I'd want `TcRn a -> TcRn (Messages, a)` + isEmptyMessages but the existing combinators 
return `TcRn a -> TcRn (Messages, Maybe a)`. I'm not at all happy with getting Nothing - even if 
renaming fails my function has to return a result.

Jan