Change jtag_add_pathmove to set jtag_error rather than call exit():

- Add new error codes to encode the possible failure conditions.
- Add documentation to describe the routine's possible error codes.


git-svn-id: svn://svn.berlios.de/openocd/trunk@2138 b42882b7-edfa-0310-969c-e2dbd0fdcd60