Error handling and alerting for MURs that are missing documents

[JonellaCulmer]

What we're after:
To catch missing MUR documents in the future, we need to incorporate better error handling and alerting when an error occurs in production. We currently get emails to the database team, but adding slack would put more eyes on this high-impact issue.

For more details:
This is related to #4361

Completion criteria:

• When reloading MURs, API should show a warning message when there are missing documents.
• Celery should send a Slack alert once a day when there are missing documents in production.

[patphongs]

Moving this for now so we can do the Slack alerting later

[patphongs]

Moved to 15.6 due to resource constraints

[lbeaufort]

Partially addressed by #4957 - we need to have a larger conversation about the best technical approach to these alerts. After checking with @fec-jli and @patphongs I'm going to put this back in "new issues" and re-prioritize this work if it the issue re-occurs.

[pkfec]

Case document refresh window has been updated in this PR #5241

Slack alerts have been setup on legal-source and bots` channel when legal documents are updated.