Improve url-parsing error messages

[hemidactylus]

They now provide guidance on the expected format:

ValueError: Cannot parse the supplied API endpoint (q). The endpoint must be in the following form: "https://<hexadecimal db uuid>-<db region>.apps.astra.datastax.com".

ValueError: Cannot parse the supplied API endpoint (q). The endpoint must be in the following form: "http[s]://<domain name or IP>[:port]".