Ensure `RESOURCE_PACK` is a URL

### Enhancement Type

Improve an existing feature

### Describe the enhancement

It is too tempting to set `RESOURCE_PACK` to a file situated on the server rather than a URL. As such, it would be helpful if the script logged a warning (or failed with an error?) if a non-URL is specified.

Start by locating the startup script or entry point that reads RESOURCE_PACK and inspect how its value is currently handled. Define done as detecting a non-URL value and consistently logging a warning or failing with an error, then run the relevant startup checks for both URL and non-URL values.

Written by the indexing model from the issue text.