Fix an ALTER GROUP ... DROP USER error message.

Fix an ALTER GROUP ... DROP USER error message.

This error message stated the privileges required to add a member
to a group even if the user was trying to drop a member:

	postgres=> alter group a drop user b;
	ERROR:  permission denied to alter role
	DETAIL:  Only roles with the ADMIN option on role "a" may add members.

Since the required privileges for both operations are the same, we
can fix this by modifying the message to mention both adding and
dropping members:

	postgres=> alter group a drop user b;
	ERROR:  permission denied to alter role
	DETAIL:  Only roles with the ADMIN option on role "a" may add or drop members.

Author: ChangAo Chen
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/tencent_FAA0D00E3514AAF0BBB6322542A6094FEF05%40qq.com
Backpatch-through: 16