Added missing variable assignments

- The "status" variable was not assigned when a command was not
  recognized or came out of sequence. This has been corrected, and the
  unit tests have been amended.
Improved SubprocessDeliveryHandler documentation

- Added documentation and usage examples to the
  SubprocessDeliveryHandler class.
- Made the "directory" argument for SubprocessDeliveryHandler default to
  the current working directory.
- Minor refactoring of exceptions, return values and variable names