shiny/driver/x11driver: add more context to shm error messages.
shiny/driver/x11driver: add more context to shm error messages.

Change-Id: Ie9ab42dcfce200e202ac4e11dfa24b743f12e649
Reviewed-on: https://go-review.googlesource.com/19215
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>

AkihiroSuda/golang-exp-aspectgo,golang/exp,golang/exp,AkihiroSuda/golang-exp-aspectgo,golang/exp,AkihiroSuda/golang-exp-aspectgo