DEVPROD-8819: make more specific host creation failure event for spawn host outcome

[Kimchelly]

DEVPROD-8819

Description

The user subscription for previously listened for host creation errors in general, which would notify them when their spawn host fails to start up (e.g. due to insufficient capacity). However, the way the notification infrastructure works that means all host creation errors have to be processed for notifications, even for non-spawn-hosts. This can put strain on the notification system when lots of hosts are failing to start (e.g. because many task hosts are hitting insufficient capacity).

Since we only want to process notifications for spawn hosts failing to be created, I made a more specific error message.

Testing

• Added unit tests for triggers.
• Tested in staging that starting a spawn host (and manually hard-coding an error during host creation) sent me a Slack notification about it failing to start up.

Documentation

N/A