cmd/vet: clean up printing errors with no position

cmd/vet: clean up printing errors with no position

Before:

: runtime/asm_amd64.s:345: [amd64] morestack: use of 8(SP) points beyond argument frame

After:

runtime/asm_amd64.s:345: [amd64] morestack: use of 8(SP) points beyond argument frame

Updates #11041

Change-Id: Ic87a6d1a2a7b2a8bf737407bc981b159825c84f2
Reviewed-on: https://go-review.googlesource.com/27152
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>