[commands] Re-order error handler arguments.

They now have Context as the first argument to be consistent with other
context-passing functions.

discord/ext/commands/bot.py

@@ -740,13 +740,13 @@ class BotBase(GroupMixin):
             try:
                 yield from ctx.command.invoke(ctx)
             except CommandError as e:
-                yield from ctx.command.dispatch_error(e, ctx)
+                yield from ctx.command.dispatch_error(ctx, e)
             else:
                 ctx.command_failed = False
                 self.dispatch('command_completion', ctx)
         elif ctx.invoked_with:
             exc = CommandNotFound('Command "{}" is not found'.format(ctx.invoked_with))
-            self.dispatch('command_error', exc, ctx)
+            self.dispatch('command_error', ctx, exc)
 
     @asyncio.coroutine
     def process_commands(self, message):