cmd/go/internal/modfetch: return structured errors from proxy operations

cmd/go/internal/modfetch: return structured errors from proxy operations

CL 181881 added structured error types for direct fetches.
Use those same structured errors to format proxy errors consistently.

Also ensure that an empty @v/list is treated as equivalent to the module
not existing at all.

Updates #27173
Updates #32715

Change-Id: I203fd8259bc4f28b3389745f1a1fde936b0fa24d
Reviewed-on: https://go-review.googlesource.com/c/go/+/183619
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>