SubjectAdder.downloadMissingImages(): Don't try if there is no network.

We already ignore the exception in the caller, because it is normal,
though we didn't actually throw the exception.
The lack of a check here might have caused us to download missing images,
though not new images, while not on wifi even while the wifi-only
setting was on.

app/src/main/java/com/murrayc/galaxyzoo/app/syncadapter/SubjectAdder.java

@@ -63,6 +63,8 @@ public SubjectAdder(final Context context, final RequestQueue requestQueue) {
     boolean downloadMissingImages() {
         boolean noWorkNeeded = true;
 
+        throwIfNoNetwork();
+
         //Get all the items that have an image that is not yet fully downloaded:
         final ContentResolver resolver = getContext().getContentResolver();