improve 'port occupied' error message`

[rrayst]

The error message

Membrane is configured to open port %d. But this port is alreay in
use by a different program.

can be confusing, as it is also printed when Membrane (the JVM process) is not allowed to open the port (e.g. when running as non-root).

(Solution might be setcap cap_net_bind_service+epi /path/to/java)