Make a new error for missing Piet SharedState.

PiperOrigin-RevId: 291008333
Change-Id: I65f1e7cc8777f0e1cc07f9c7d68e68706b858839
diff --git a/src/main/proto/search/now/ui/piet/errors.proto b/src/main/proto/search/now/ui/piet/errors.proto
index ec43a36..4171443 100644
--- a/src/main/proto/search/now/ui/piet/errors.proto
+++ b/src/main/proto/search/now/ui/piet/errors.proto
@@ -132,6 +132,17 @@
   // Fields start at ID 1.
   // ---------------------------------------------------------------------------
 
+  // When the client tries to reference a SharedState that is not found, the
+  // Frame cannot be rendered, and likely no frames can be rendered.
+  // This error code must be reported by the Piet host app, as the Piet
+  // implementation assumes that all shared states have been provided already.
+  ERR_MISSING_SHARED_STATE = 12 /* [
+    // Something is seriously wrong if a SharedState is missing
+    (server_error) = FATAL,
+    // Clients cannot render any Frame when SharedState is missing (no Template)
+    (client_error) = FATAL
+  ] */;
+
   // When a Template cannot be located, it only affects Frames that reference
   // it, so we can proceed to render other unaffected Frames, making this an
   // ERROR, not FATAL.