Add docstrings for ErrorEvent fields (#13)

Author: vrslev

stompman/listening_events.py

@@ -59,7 +59,9 @@ async def with_auto_ack(
 @dataclass
 class ErrorEvent:
     message_header: str = field(init=False)
+    """Short description of the error."""
     body: bytes = field(init=False)
+    """Long description of the error."""
     _frame: ErrorFrame
     _client: "Client" = field(repr=False)