go/types, types2: use zero error code to indicate unset error code

go/types, types2: use zero error code to indicate unset error code

Use InvalidSyntaxError where the zero error code was used before.
Fix a couple of places that didn't set an error code.
Panic in error reporting if no error code is provided.

Change-Id: I3a537d42b720deb5c351bf38871e04919325e231
Reviewed-on: https://go-review.googlesource.com/c/go/+/439566
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>