[Features] Fallback to the global configuration if credentials are invalid

[josuemontano]

Is your feature request related to a problem? Please describe.
If one forgets to enter the right MagicBell credentials, people might be worried which account the notifications would go out on.

Describe the solution you'd like
Fallback to the global configuration if the given credentials are invalid.

Describe alternatives you've considered
Raise an error if the given credentials are invalid.