use-debugger (&optional flag)

By default, errors that occur while running tests are simply counted and ignored. You can change this behavior by calling use-debugger with one of three possible flag values: t (the default) means your Lisp's normal error handling routines will be invoked when errors occur; :ask means you will be asked what to do when an error occurs, and nil means errors are counted and ignored, i.e., the standard behavior.