[commands] Copy on_error handlers in Command.copy

This fixes the issue of error handlers not applying.

discord/ext/commands/core.py

@@ -270,6 +270,10 @@ class Command(_BaseCommand):
         ret = self.__class__(self.callback, **self.__original_kwargs__)
         ret._before_invoke = self._before_invoke
         ret._after_invoke = self._after_invoke
+        try:
+            ret.on_error = self.on_error
+        except AttributeError:
+            pass
         return ret
 
     def _update_copy(self, kwargs):