moves handling of problems with resetting into the halted state

into the target implementation.

Also target_process_reset() is now simpler and has error handling,
e.g. if assert reset fails, then target_process_reset() will propagate
that error.

cmd_ctx was passed in to examine(), which is wrong - removed that.

git-svn-id: svn://svn.berlios.de/openocd/trunk@887 b42882b7-edfa-0310-969c-e2dbd0fdcd60