Exit gracefully when socket fails to open

This fixes prevents a traceback that was shown when trying to listen to
an address on which 'bind' fails.

websockify/websockifyserver.py

@@ -718,6 +718,7 @@ class WebSockifyServer():
         be overridden) for each new client connection.
         """
 
+        try:
             if self.listen_fd != None:
                 lsock = socket.fromfd(self.listen_fd, socket.AF_INET, socket.SOCK_STREAM)
             elif self.unix_listen != None:
@@ -732,6 +733,10 @@ class WebSockifyServer():
                                     tcp_keepcnt=self.tcp_keepcnt,
                                     tcp_keepidle=self.tcp_keepidle,
                                     tcp_keepintvl=self.tcp_keepintvl)
+        except OSError as e:
+            self.msg("Openening socket failed: %s", str(e))
+            self.vmsg("exception", exc_info=True)
+            sys.exit()
 
         if self.daemon:
             keepfd = self.get_log_fd()