Mattermost is sent by the Slack sender, which wrapped every HTTP error
status in ErrSlackFailed, so a Mattermost endpoint answering 503 was
logged as "slack notification failed". The sender now takes the error
to wrap: the Slack endpoint passes ErrSlackFailed and the Mattermost
endpoint passes ErrMattermostFailed, which was defined but unused.
A new delivery test sets both endpoints to a stand-in server answering
503 and checks the error logged for each names its own endpoint.
Model: opus-5-5
sneak