ajp: Changes to acq saving and error handling.

Three main changes:
1. Acq objects are now only saved if a processing step executed without
error, not if it aborted with error.
2. If an error happens, the acq object is now moved to a dedicated
"error" directory, not back to the toDo directory. Moving it back to
toDo could cause infinite loops of processing error-causing files.
3. The function to save the current acquisition was turned into an
ajp-object method.