If no error handler is provided, terminate parsing with first error
and report that error.

Fixes #17697.

Change-Id: I9070faf7239bd53725de141507912b92ded3474b
Reviewed-on: https://go-review.googlesource.com/32456
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>