Warn when stream is closing due to error, not debug. (#1561)

Author: wu-hui

firebase-firestore/src/main/java/com/google/firebase/firestore/remote/AbstractStream.java

@@ -142,7 +142,7 @@ public void onClose(Status status) {
                   "(%x) Stream closed.",
                   System.identityHashCode(AbstractStream.this));
             } else {
-              Logger.debug(
+              Logger.warn(
                   AbstractStream.this.getClass().getSimpleName(),
                   "(%x) Stream closed with status: %s.",
                   System.identityHashCode(AbstractStream.this),