errorTypeName method has sensible default implementation

[parsonsmatt]

Using DefaultSignatures, the errorTypeName method could easily be given the implementation:

    default errorTypeName :: (Typeable a) => Proxy a -> String
    errorTypeName prxy = show (typeRep prxy)

Since all types are Typeable in recent GHC, this would be trivially satisfiable for all types.

[parsonsmatt]

This is a somewhat larger change than anticipated, since we are using CPP to conditionally check on the availability of DefaultSignatures. This is to support GHC < 7.2.

I can make a PR keeping GHC < 7.2 support that uses CPP in all of the instance declarations to give Typeable constraints if DefaultSignatures is available, and not otherwise. Or we can drop support for < 7.2 and remove the CPP entirely. I'm weakly in favor of the latter option as it would simplify the library, and I'd be shocked if anyone is still on GHC 7.0 or earlier and using new versions of this package.