Add _urlIsWhitelisted to notifier

If hostWhiteList config option is set, check the payload’s frames and
filenames to verify that they have been whitelisted. Ignores “unknown”
filenames.

Add missing UNKNOWN_FUNCTION and computeStackTraceWrapper
They were not fully cleared from the codebase, so adding them back in
from a previous beta release for now.

Add missing setupCustomJSON

Pulled from previous release. Just copy/paste without looking. Living
on the edge. More specs passing because of this.

Expand on "unknown error" message.