Raise helpful error when role doesn't exist

If you try to call `connected_to` with a role that doesn't have an
established connection you used to get an error that said:

```
>> ActiveRecord::Base.connected_to(role: :i_dont_exist) { Home.first }

ActiveRecord::ConnectionNotEstablished Exception: No connection pool
with 'primary' found.
```

This is confusing because the connection could be established but we
spelled the role wrong.

I've changed this to raise if the `role` used in `connected_to` doesn't
have an associated handler. Users who encounter this should either check
that the role is spelled correctly (writin -> writing), establish a
connection to that role in the model with connects_to, or use the
`database` keyword for the `role`.

I think this will provide a less confusing error message for those
starting out with multiple databases.

0 files changed, 0 insertions, 0 deletions