[commands] ColourConverter raises if value is out of range

changes make the `ext.commands.ColourConverter` Converter fail when
user input is outside the acceptable value range 0x000000 - 0xFFFFFF

discord/ext/commands/converter.py

@@ -294,6 +294,8 @@ class ColourConverter(Converter):
             arg = arg[1:]
         try:
             value = int(arg, base=16)
+            if not (0 <= value <= 0xFFFFFF):
+                raise BadArgument('Colour "{}" is invalid.'.format(arg))
             return discord.Colour(value=value)
         except ValueError:
             method = getattr(discord.Colour, arg.replace(' ', '_'), None)