Sending memo to user without the receivememo permission should give a better error message

[marcog]

17:15 <%tumbleweed> tibid: tell Taejo test
17:15 < tibid> tumbleweed: Just tell Taejo yourself
17:16 <%tumbleweed> tibid: permissions for taejo
17:16 < tibid> tumbleweed: Permissions: -receivememo, accounts, admin, config,
               core, debug, eval, factoidadmin, karmaadmin, plugins, recvmemo,
               saydo and sources

The response should be more informative, such as "I can't deliver to Taejo".