go/types, types2: call error handler for each sub-error as needed

go/types, types2: call error handler for each sub-error as needed

Factor out calling or typechecker error handler from error_.report.
In error_.report, decide if the typechecker error handler needs to
be called once or multiple times.

This change enables the use of sub-errors for types2 and go/types,
with the error handler taking care of deciding how many "separate"
errors are reported via the API.

Use new error reporting in go/types mono and initorder computation;
with the above adjustments, these changes should now pass gopls tests.

Also: adjust some format strings to avoid vet errors.

Change-Id: If05a7044399b4783c596c69a8158619f83c21c70
Reviewed-on: https://go-review.googlesource.com/c/go/+/566537
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>