Return `E_NOT_SUPPORTED` from slave-only calls on master

This commit changes the error returned from `_not_if_master` and `quiesce_db`
from `E_UNKNOWN_FAILURE` (which closes the client socket) to `E_NOT_SUPPORTED`
(which keeps the client socket as-is) in case the call is only valid on
non-master nodes.
Merge pull request #268 from Incubaid/use-not-supported

Return `E_NOT_SUPPORTED` from slave-only calls on master