Fix registration without force (whisperfish#192)

In the presage-store-sled implementation, `load_state` always returns
`Ok(_)` (except for errors). Therefore, the registration condition that
determines if the store was already registered, `!force &&
config_store.load_state.is_ok()` is always `true` when `force == false`.
Instead, one should use `config_store.is_registered()` to figure out if
the store is already registered.

presage/src/manager.rs

@@ -188,7 +188,7 @@ impl<C: Store> Manager<C, Registration> {
         } = registration_options;
 
         // check if we are already registered
-        if !force && config_store.load_state().is_ok() {
+        if !force && config_store.is_registered() {
             return Err(Error::AlreadyRegisteredError);
         }