When using rr.serve and specifying ports, they are chosen at random if they are already occupied.

[jleibs]

Describe the bug
Ports are chosen at random when they are already occupied.

To Reproduce
Script

#!/usr/bin/env python3
import rerun as rr
import time

def main() -> None:
    rr.init("rerun_example_serve")
    rr.serve(web_port=57158, ws_port=57159)

    try:
        while True:
            time.sleep(1)
    except KeyboardInterrupt:
        print("Ctrl-C received. Exiting.")

if __name__ == "__main__":
    main()

Run the program twice from two separate terminals

Run1

➜  python spawn_ports.py 
[2024-05-03T22:30:55Z INFO  re_ws_comms::server] Hosting a WebSocket server on ws://localhost:57159. You can connect to this with a native viewer (`rerun ws://localhost:57159`) or the web viewer (with `?url=ws://localhost:57159`).
[2024-05-03T22:30:55Z INFO  re_web_viewer_server] Started web server on http://localhost:57158
[2024-05-03T22:30:55Z INFO  re_sdk::web_viewer] Hosting a web-viewer at http://localhost:57158?url=ws://localhost:57159
^CCtrl-C received. Exiting.
[2024-05-03T22:31:03Z INFO  re_ws_comms::server] Shutting down Rerun server on ws://localhost:57159
[2024-05-03T22:31:03Z INFO  re_web_viewer_server] Shutting down web server on http://localhost:57158

Run 2

➜  python spawn_ports.py 
[2024-05-03T22:31:00Z INFO  re_ws_comms::server] Hosting a WebSocket server on ws://localhost:42271. You can connect to this with a native viewer (`rerun ws://localhost:42271`) or the web viewer (with `?url=ws://localhost:42271`).
[2024-05-03T22:31:00Z INFO  re_web_viewer_server] Started web server on http://localhost:33623
[2024-05-03T22:31:00Z INFO  re_sdk::web_viewer] Hosting a web-viewer at http://localhost:33623?url=ws://localhost:42271
^CCtrl-C received. Exiting.
[2024-05-03T22:31:03Z INFO  re_ws_comms::server] Shutting down Rerun server on ws://localhost:42271
[2024-05-03T22:31:03Z INFO  re_web_viewer_server] Shutting down web server on http://localhost:33623

Expected behavior
The second run should fail with a warning about the ports being occupied.

Desktop (please complete the following information):

• OS: Linux

Rerun version
0.15.1