Better error when SMTP is not configured

[mattttt]

When adding an Email Notification Channel and SMTP is not configured, the interface throws this error:

This is not a helpful msg, and doesnt give the user any next steps to try and address the issue. The Grafana logs say:

EROR[04-11|16:24:51] Failed to send alert notifications       logger=context userId=1 orgId=1 uname=admin error="Grafana mailing/smtp options not configured, contact your Grafana admin"

however unless you're looking at the server logs, you'd never know it.

Proposed solution

When configuring alerts, do a check to see if SMTP is configured. If not, show a warning box within the form flow stating "Grafana mailing/SMTP options have not been configured for your instance. Ask your Grafana admin to set this up."