We assumed that in the not-found case GetMutableTableByName would
return a nil error. But, in fact, it returns a non-nil error, causing
us to fail when building our list of optOut system tables.

Now, we look for the DescriptorNotFound error and ignore it.

We could have alternatively built this list of descriptors using the
package-level vars in the catalog/systemschema package. We opted
against that originally since we wanted to stop writing code that
assumes static system table IDs. I've kept with that same decision
here.

Release note (enterprise change): BACKUP WITH revision_history would
previously fail on an upgraded but un-finalized cluster. Now, it
should succeed.