[commands] Add a converter for discord.Object

discord/ext/commands/errors.py

@@ -42,6 +42,7 @@ __all__ = (
     'MaxConcurrencyReached',
     'NotOwner',
     'MessageNotFound',
+    'ObjectNotFound',
     'MemberNotFound',
     'GuildNotFound',
     'UserNotFound',
@@ -213,6 +214,23 @@ class NotOwner(CheckFailure):
     """
     pass
 
+class ObjectNotFound(BadArgument):
+    """Exception raised when the argument provided did not match the format
+    of an ID or a mention.
+
+    This inherits from :exc:`BadArgument`
+
+    .. versionadded:: 2.0
+
+    Attributes
+    -----------
+    argument: :class:`str`
+        The argument supplied by the caller that was not matched
+    """
+    def __init__(self, argument):
+        self.argument = argument
+        super().__init__(f'{argument!r} does not follow a valid ID or mention format.')
+
 class MemberNotFound(BadArgument):
     """Exception raised when the member provided was not found in the bot's
     cache.