Don't create impl candidates when obligation contains errors

Don't create impl candidates when obligation contains errors

Fixes #72839

In PR #72621, trait selection was modified to no longer bail out early
when an error type was encountered. This allowed us treat `ty::Error` as
`Sized`, causing us to avoid emitting a spurious "not sized" error after
a type error had already occured.

However, this means that we may now try to match an impl candidate
against the error type. Since the error type will unify with almost
anything, this can cause us to infinitely recurse (eventually triggering
an overflow) when trying to verify certain `where` clauses.

This commit causes us to skip generating any impl candidates when an
error type is involved.