cmd/compile/internal/syntax: better error message for erroneous method declaration

cmd/compile/internal/syntax: better error message for erroneous method declaration

Also make error recovery slightly more robust in this case.

Fixes #56022.

Change-Id: I1c01c1465adb48c71367d037b6f0e3fe56f68ec9
Reviewed-on: https://go-review.googlesource.com/c/go/+/438540
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>