Combine error checks

https://github.com/Graylog2/collector-sidecar/blob/v2/superv/supervisor/supervisor.go#L785-L791

Since both of these need to be set, it would be nice if the error checking/reporting was also combined and not one after the other.

Open superv/supervisor/supervisor.go at lines 785-791 and inspect the two error checks and their reporting. Confirm how both required values are checked, then combine the checks and run the relevant project tests. Done means the required checks are reported together rather than one after the other.

Written by the indexing model from the issue text.