[Explore sites]: Update error page text to be more compliant

Bug: 916771
Change-Id: Ibd0e487410fe5f0c839a3e76582caab8ccf15fe3
Reviewed-on: https://chromium-review.googlesource.com/c/1435474
Auto-Submit: Cathy Li <chili@chromium.org>
Reviewed-by: Peter Williamson <petewil@chromium.org>
Commit-Queue: Peter Williamson <petewil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625945}