Flag server messages so AnkiDroid can handle them separately

proto/anki/backend.proto

@@ -30,6 +30,7 @@ message BackendError {
     DB_ERROR = 5;
     NETWORK_ERROR = 6;
     SYNC_AUTH_ERROR = 7;
+    SYNC_SERVER_MESSAGE = 23;
     SYNC_OTHER_ERROR = 8;
     JSON_ERROR = 9;
     PROTO_ERROR = 10;

rslib/src/backend/error.rs

@@ -66,6 +66,7 @@ impl From<SyncErrorKind> for Kind {
     fn from(err: SyncErrorKind) -> Self {
         match err {
             SyncErrorKind::AuthFailed => Kind::SyncAuthError,
+            SyncErrorKind::ServerMessage => Kind::SyncServerMessage,
             _ => Kind::SyncOtherError,
         }
     }