app: improve region-locked error text and status code
app: improve region-locked error text and status code

I made a similar change for the Share button in golang.org/cl/25541 but
didn't add it on the read side.

This also improves the error text with actionable advice.

Updates golang/go#18488.
Updates golang/go#16617.

Change-Id: Iee136c22c4e0fe52c302354123ea2baae58cfb4c
Reviewed-on: https://go-review.googlesource.com/34829
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/playground,golang/playground,golang/playground