compiler: improve error for import of non-string

Change-Id: I97fc28fc21166e024cb5906da894075f8315669b
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/273867
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
1 file changed