Fix wrapped unknown types

[maartenvanvliet]

I noticed that wrapped unknown types were not marked as errors by the Absinthe.Phase.Validation.KnownTypeNames phase.

E.g.

query Foo($var: UnknownType) { user { id } }

would trigger an error but

query Foo($var: UnknownType!) { user { id } }

wouldn't.
Downstream in other phases this error would still be picked up because e.g. the variable type would not match up to the argument, but the error message was vague.

I added a fix for this and also changed the error message to include "Did you mean?" suggestions.