Add `Success` prefix to successful email notifications

[yruslan]

Background

This is so they can be easily filtered into a special folder.

Feature

Add Success: prefix to successful email notifications.

Proposed Solution

Distinguish:

• Success - when no issues
• Warning - when there is a warning or a schema change
• Partial success - when some tasks succeeded and some failed
• Failure - when the pipeline has failed or all tasks have failed