Clean up error message for hardAssert based on peer feedback.

Author: MarkDuckworth

packages/firestore/src/remote/watch_change.ts

@@ -242,9 +242,10 @@ class TargetState {
     this.pendingResponses -= 1;
     hardAssert(
       this.pendingResponses >= 0,
-      'Unexpected state, `pendingResponses` is less than 0. This ' +
-        'indicates that the SDK received more target acks from the server than ' +
-        'expected. The SDK should not continue to operate.'
+      '`pendingResponses` is less than 0. Actual value: ' +
+        this.pendingResponses +
+        '. This indicates that the SDK received more target acks from the ' +
+        'server than expected. The SDK should not continue to operate.'
     );
   }