Update the call order and allow called functions to exit

* hydra.el (defhydra): First disable the transient map, then call red
  head, allowing it to throw `hydra-disable' to break, then re-set
  transient map.

  If the called function raises an error, display this error for a
  while, but still set the transient map.

* hydra-test.el: Update test.

Re #15.

Author: abo-abo