register_server: Improve UX with the "hostname already in use" error.

An even better way than the current json error message recommending the
--registration-transfer option is to return an appropriate error code
and have that get picked up by the register_server command.

The register_server command can then display a more comprehensive,
better formatted error message with proper whitespaces and a pointer to
the documentation.

zerver/lib/push_notifications.py

@@ -1528,3 +1528,19 @@ class InvalidRemotePushDeviceTokenError(JsonableError):
 class PushNotificationsDisallowedByBouncerError(Exception):
     def __init__(self, reason: str) -> None:
         self.reason = reason
+
+
+class HostnameAlreadyInUseBouncerError(JsonableError):
+    code = ErrorCode.HOSTNAME_ALREADY_IN_USE_BOUNCER_ERROR
+
+    data_fields = ["hostname"]
+
+    def __init__(self, hostname: str) -> None:
+        self.hostname: str = hostname
+
+    @staticmethod
+    @override
+    def msg_format() -> str:
+        # This message is not read by any of the client apps, just potentially displayed
+        # via server administration tools, so it doesn't need translations.
+        return "A server with hostname {hostname} already exists"