gl_server should abort with an error message if the port is already in use.

[coil-lighting]

If I try to run two copies of gl_server, or if I run a conflicting server on the same point (e.g. the https://github.com/patternleaf/archway OPC server) and then start gl_server, the second copy should abort with a helpful error message when it discovers the address:port endpoint is already taken by the previous process.

Instead it behaves as if nothing is wrong, even though it cannot actually accept clients.