feat(server::client): proper connect error

protobufs/client.proto

@@ -24,9 +24,19 @@ message ClientRequest {
   }
 }
 
+message ClientConnectError {
+  enum Code {
+    UNKNOWN = 0;
+    APPROVAL_DENIED = 1;
+    NO_USER_AGENTS_ONLINE = 2;
+  }
+  Code code = 1;
+}
+
 message ClientResponse {
   oneof payload {
     AuthChallenge auth_challenge = 1;
     AuthOk auth_ok = 2;
+    ClientConnectError client_connect_error = 5;
   }
 }