Better error message on real-time when it closes for duplicate topic.

[dshukertjr]

Is your feature request related to a problem? Please describe.
Currently when a user tries to listen to real-time events using realtime-dart, the connection closes if the user has already opened a connection with the same topic. When this happens, there are no intuitive error message or anything given. Because of this, first time users of supabase struggle to find the cause of the issue.

Describe the solution you'd like
Give some kind of error message maybe as a callback of subscribe? I haven't put to much thought into how it should look at the end.

Describe alternatives you've considered
Add docs?

Additional context
Just jotting some thoughts before I forget it.

[Vinzent03]

In a short test, I encountered that both channels were closed. Even with delay between the two channel creations, both were closed and non received events. May create a pr with the test.