[mlir] Add a better error message when failing to parse an attribute

The fallback attribute parse path is parsing a Type attribute, but this results
in a really unintuitive error message: `expected non-function type`, which
doesn't really hint at tall that we were trying to parse an attribute. This
commit fixes this by trying to optionally parse a type, and on failure
emitting an error that we were expecting an attribute.

Differential Revision: https://reviews.llvm.org/D124870