try to recover the non-matching types in projection errors

try to recover the non-matching types in projection errors

The type equation in projection takes place under a binder and a snapshot, which
we can't easily take types out of. Instead, when encountering a projection error,
try to re-do the projection and find the type error then.

This fails to produce a sane type error when the failure was a "leak_check" failure.
I can't think of a sane way to show *these*, so I just left them use the old crappy
representation, and added a test to make sure we don't break them.