Raise error if deffered and already responded to

Added check to defer()
Added AlreadyResponded error

Author: quirky-bluejay

discord_slash/context.py

@@ -86,6 +86,8 @@ async def defer(self, hidden: bool = False):
 
         :param hidden: Whether the deffered response should be ephemeral . Default ``False``.
         """
+        if self._deffered or self._sent:
+            raise error.AlreadyResponded("You have already responded to this command!")
         base = {"type": 5}
         if hidden:
             base["data"] = {"flags": 64}

discord_slash/error.py

@@ -54,3 +54,8 @@ class IncorrectType(SlashCommandError):
     """
     Type passed was incorrect
     """
+
+class AlreadyResponded(SlashCommandError):
+    """
+    The interaction was already responded to
+    """