peel: reject bad queries with EPEEL

There are some combination of objects and target types which we know
cannot be fulfilled. Return EPEEL for those to signify that there is a
mismatch in the user-provided data and what the object model is capable
of satisfying.

This should resolve #2627.
That is, the object model does not allow (comimt|tree) -> blob, but it does allow tag->blob which is why it doesn't return EPEEL here.