A quick errand and an urgent job are not the same thing. An errand involves taking a short trip outside. For example, collecting dry cleaning, returning books to the library, or going to a store to buy milk are all errands. The word has no suggestion of urgency. A job can be done anywhere. It might involve taking a short trip outside, or it might involve sitting at your desk. You could say: I have to go out I have to go somewhere I have to so something

Depending on the situation, but maybe "i'd have to be away for a sec!" :)