Get a decent MatchContext for pattern synonym bindings

In particular when we have a pattern binding
    K p1 .. pn = rhs
where K is a pattern synonym.  (It might be nested.)

This small MR fixes #25995.  It's a tiny fix, to an error message,
removing an always-dubious `unkSkol`.

The bug report was in the context of horde-ad, a big program,
and I didn't manage to make a small repro case quickly. I decided
not to bother further.