[Event Bus] Implement sad-path for simple Studio publish events

[robrap]

This epic is part of the Event Bus work documented in openedx/platform-roadmap#28.

The epic covers the implementation of the sad-paths for a new use case. The current plan is to implement the use case of course publishing from Studio that could be consumed by Discovery, potentially replacing a small piece of the Refresh Course Metadata batch job. For the purposes of this Epic, we will handle any error or exceptional conditions. This may include:

• Publishing cached events on server shutdown.
• Error handling while publishing events.
• Error handling while consuming events.
  • Likely to include a Dead Letter Queue for events that can't be processed.

The code developed during this epic will have an eye toward reusability and pluggability, but will err toward getting something worked so that the API can be defined in a later iteration.

Milestone Details:

• Title: [Event Bus] Implement sad-path
• Description: See https://github.com/openedx/openedx-events/issues/90 for Epic details.
• Due Date: TBD
  • Important: Date changes need to be fixed in all repos with this milestone defined.

Open issues/PRs with this milestone: https://github.com/pulls?q=is%3Aopen+milestone%3A%22%5BEvent+Bus%5D+Implement+sad-path%22

[robrap]

This has been completed.