Re: datachecksums: handle invalid and dropped databases during enable

> Apologies if this is expected behaviour, and I missed on some thread
> mentioning it.

It was somewhat intentional, but I agree that it is inconsistent and
maybe we should improve it.

The original code wanted to skip invalid databases, but also failed
because it tried connecting to them before this check. I fixed the
connection failure, but I also added a startup check about having no
invalid databases, because completing checksum enablement while there
are invalid databases didn't seem entirely safe. This seemed like a
good compromise because I wanted to avoid surprise process stops, but
since a failing drop database happening during the processing is
unlikely, handling this properly shouldn't be a real problem.

We could improve this either by adding your patch, or by removing
ErrorOnInvalidDatabases, but adding this patch seems to be safer to
me.