In Webhooks, give errors human-readable labels and show reminder text for "Silent Mode"

Summary:
Depends on D19928. See https://discourse.phabricator-community.org/t/firehose-webhook-not-working-with-self-hosted-requestbin-instance/2240/.

Currently, we report "hook" and "silent", which are raw internal codes.

Instead, report human-readable labels so the user gets a better hint about what's going on ("In Silent Mode").

Also, render a "hey, you're in silent mode so none of this will work" reminder banner in this UI.

Test Plan:

Note:

• New warning banner.
• Table has more human-readable text ("In Silent Mode").

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19929