go/importer: better error message when importer is out of date

Separated out panic handling for bimporter and importer so that
the handler can consider the current version and report a better
error.

Added new export data test for export data version 999 (created
by changing the compiler temporarily) and verifying expected
error message.

Fixes #25856.

Change-Id: Iaafec07b79499154ef7c007341783fa07c57f24d
Reviewed-on: https://go-review.googlesource.com/118496


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>