go/types, types2: use "invalid operation: x rel y (cause)" for comparison error messages

go/types, types2: use "invalid operation: x rel y (cause)" for comparison error messages

Matches compiler behavior and is consistent with what we do with other
binary operations.

While at it, also use parentheses rather than a colon for a couple of
errors caused by not having a core type.

For #55326.

Change-Id: I0a5cec1a31ffda98d363e5528791965a1ccb5842
Reviewed-on: https://go-review.googlesource.com/c/go/+/435618
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>