fail for two of it's return codes. A little bit weird, but compatible with
  existing codebase.
- tightend up error handling. Since the jtag_xxx() is a queue that is either 
  executed as things are added(hw queue) or a software queue, then
  errors can only be caught during jtag_execute_queue(). No error
  code is therefore returned from the queuing fn's. 

git-svn-id: svn://svn.berlios.de/openocd/trunk@492 b42882b7-edfa-0310-969c-e2dbd0fdcd60