Make this error fail gracefully: Email Reports after a Custom Report gets deleted

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(The fix for this needs to be added in core, since Email reports are part of core. - Altamash)

Email Report breaks if you delete a Custom Report in it

Problem: If you have an Email Report that contains a deleted Custom Report, the email fails with an error in the Admin Dashboard (when you try to save the email settings).

Solution: It seems reasonable to instead make the Email Report fail silently / gracefully.

Steps to reproduce

1. Create an Email Report
2. Put one or more Custom Reports into the Email Report
3. Save
4. Go to Admin >> Custom Reports. Delete one or more Custom Reports
5. Open the email report settings
6. Save
7. Fails with the following message:

Report CustomReports_getCustomReport_idCustomReport--3 is unknown or not available for report type 'email'. (when I did it)
Report CustomReports_getCustomReport_idCustomReport--64 is unknown or not available for report type 'email'. (when the user did it)

Who has produced this error:

• me
• the user who reported it
• Altamash

Workaround:

1. Person who reported this went into database and did a SQL query, reactivated the deleted Custom Report.
2. Once it was reactivated in the db, it became visible in Email Report.
3. Then they solved the problem by simply unchecking this Custom Report, while in Email Report settings.

Prognosis:
As there exists a workaround, where user can re-create the same report fresh (without the bug), it’s not a “blocking” issue. We’ll fix it eventually but not high priority.