fix: gracefully handle port-in-use error on server startup

Catch EADDRINUSE OSError when binding the TCP site and exit with a
clear error message instead of an unhandled traceback.

Amp-Thread-ID: https://ampcode.com/threads/T-019cf85a-650a-73fa-8201-1dbddcbbd9e4
Co-authored-by: Amp <amp@ampcode.com>

server.py

@@ -1,3 +1,4 @@
+import errno
 import os
 import sys
 import asyncio
@@ -1236,7 +1237,13 @@ class PromptServer():
             address = addr[0]
             port = addr[1]
             site = web.TCPSite(runner, address, port, ssl_context=ssl_ctx)
-            await site.start()
+            try:
+                await site.start()
+            except OSError as e:
+                if e.errno == errno.EADDRINUSE:
+                    logging.error(f"Port {port} is already in use on address {address}. Please close the other application or use a different port with --port.")
+                    raise SystemExit(1)
+                raise
 
             if not hasattr(self, 'address'):
                 self.address = address #TODO: remove this