Warn on pg:psql with unattached credential

[msakrejda]

If you try to run heroku pg:psql --credential unattached-cred (where unattached-cred exists but is not attached anywhere), you'll get the opaque error

Couldn't find that addon.

Which is not entirely correct (the addon may have been found, but the attachment using the specified credential was not). We should give a better error in this case.