[NTP] Support the error notification customization and move its handler to local_ntp.js

Allow the error notification content to be customized by the caller,
and moved the error notification handler from custom_backgrounds.js to
local_ntp.js (the notification will be reused for custom link errors).

Also adjusted the notification styling to better match spec.

Screenshots:
https://screenshot.googleplex.com/BN02EbSSKLM.png
https://screenshot.googleplex.com/KpxwyZJ4N3T.png

Bug: 874188
Change-Id: Ibf4ebed906fd36916a4f0ddb2170a2dbe8870ab3
Reviewed-on: https://chromium-review.googlesource.com/1182787
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585991}