burst_buffer/cray plugin updated for Cray UP06 sofware

burst_buffer/cray plugin modified to work with changes in Cray UP06
   software release.
Specific changes: Cray software now returns an error if a state_in
   or stage_out script is processed that doesn't actually request a
   stage in or out (previously silently ignored).
Also the warning message about tearing down a buffer that is already
   gone changed.