fix(firestore): address a missing branch in watch.stop() error remapping (#3643)

* fix(firestore): address a missing branch in watch.stop() error remapping

* fix(firestore): prefer not using else
diff --git a/firestore/watch.go b/firestore/watch.go
index 671ce9e..e4ac7fb 100644
--- a/firestore/watch.go
+++ b/firestore/watch.go
@@ -446,8 +446,11 @@
 		return
 	}
 	if err != nil {
+		// if an error occurs while closing the stream
 		s.err = err
+		return
 	}
+	// if we close successfully,
 	s.err = io.EOF // normal shutdown
 }