fix connection string error handling
fix connection string error handling

Previous fix to global connection object means might get
`ConnectionError` in any API call. Fixups accordingly.

In particular, for connection string, we should print "unable to
connect" for any error. Maybe less accurate, depending on the error,
but better for users.

uwescience/myria-web,uwescience/myria-web,uwescience/myria-web,uwescience/myria-web