A better error message attempting to use ATTACH DATABASE

[manadart]

When attempting to use ATTACH DATABASE, not supported by Dqlite, the error reports:

disk I/O error

A more descriptive error would benefit users.

[freeekanayaka]

Out of curiosity, why did you try to use ATTACH DATABASE? Do you have a use case for that or were you just playing around?

[manadart]

For Juju's use-case, we are looking at having a DB per model.

We were looking at ATTACH DATABASE under the assumption that each DB connection would consume an OS thread. We've been told since that this is not the case, so we don't actually need it.

[cole-miller]

It's possible that the same underlying problem is cropping up in #435, because one of the first things that happens in the implementation of VACUUM is an ATTACH DATABASE. In that instance the ATTACH DATABASE appears to succeed, though.