Bulk API: Fix return of wrong request type on failed updates

In case an update request failed (for example when updating with a
wrongly formatted date), the returned index operation type was index
instead of update.

Closes #6630
a bit of nitpicking here: can we use a constant? I see that we use the same string many times in this class. Same could be done for delete and the other string constants as well while we are at it (could be a separate change though)...