device: Fail with any non-zero return value on storage print_data_save()

When saving the prints we use g_file_set_contents under the hood and in
case return its error code that is a positive value.

So in such case we don't fail if we have a write failure at the end of
the enrollment.

While we could ensure in file storage to always return a negative value,
it's always better to ensure that is has to be 0 when we didn't get an
error.

Add a test checking for this case.

src/device.c

@@ -1791,7 +1791,7 @@ enroll_cb (FpDevice *dev, GAsyncResult *res, void *user_data)
     {
       int r;
       r = store.print_data_save (print);
-      if (r < 0)
+      if (r != 0)
         name = "enroll-failed";
     }