[Patch] Don't stop a `vacuumdb --all` on a connection failure

Currently, `vacuumdb --all` returns FATAL and stops if it can't connect to one of the discovered databases. On some hosted providers (in particular, CloudSQL), there are reserved databases that the credentials supplied to the user can't connect to. This makes `vacuumdb --all` less than useful in those environments.

Attached is a patch that de-escalates the error so that vacuumdb continues if it can't connect to a database. It does mean that if the connection info is fundamentally wrong, or the instance is unreachable, the command doesn't error out immediately. I don't think there's anything particularly valuable about the current behavior, so it's a general change rather than yet another switch.