[commands] Add param attribute to MissingRequiredArgument

This should allow easier querying on what argument is missing.

Fixes #470.

discord/ext/commands/core.py

@@ -226,7 +226,7 @@ class Command:
             if param.kind == param.VAR_POSITIONAL:
                 raise RuntimeError() # break the loop
             if required:
-                raise MissingRequiredArgument('{0.name} is a required argument that is missing.'.format(param))
+                raise MissingRequiredArgument(param)
             return param.default
 
         if consume_rest_is_special:

discord/ext/commands/errors.py

@@ -68,8 +68,15 @@ class CommandNotFound(CommandError):
 class MissingRequiredArgument(UserInputError):
     """Exception raised when parsing a command and a parameter
     that is required is not encountered.
+
+    Attributes
+    -----------
+    param: str
+        The argument that is missing.
     """
-    pass
+    def __init__(self, param):
+        self.param = param.name
+        super().__init__('{0.name} is a required argument that is missing.'.format(param))
 
 class TooManyArguments(UserInputError):
     """Exception raised when the command was passed too many arguments and its