[Functions] Actually return when processing an error

The old code use to throw, which stop any further processing. The
change introduced later didn't, causing errors.

Author: rlazo

firebase-functions/src/main/java/com/google/firebase/functions/PublisherStream.kt

@@ -306,6 +306,7 @@ internal class PublisherStream(
           null
         )
       )
+      return
     }
 
     val text = response.body()?.string() ?: ""