ICU-21662 Improve UVector error handling.

ICU-21662 Improve UVector error handling.

This is the next installment of UVector error handling cleanup. It includes:

- Revise UStack to follow the conventions of UVector, to leave the stack
  unmodified when there is an incoming error code. And, for stacks with a
  deleter function, to delete the incoming object if it cannot be
  succesfully pushed.

 - Review all useage of UStack in ICU; adjust call sites as needed.

 - Review all uses of UVector::addElementX() in the implementation of
   - Regular Expressions
   - Break Iteration
   - toolutil/xmlparser

   changing to the updated functions, and adjusting call sites as needed.