TODO Ensure fail fast behavior.
Adding exceptions module.
Added command interpreter modules.

Will be used for interacting with a live running daemon.
Restructured command interpreter tests that assert raises.

Previous set-up with context apparently returns OK as long as something in the
context-wrapped code raises the provided exception.
All the right exceptions in the right places.
Specified a boolean return value for interpret_command.

Also refactored objects in command interpreter tests to a setUp method. Added
mock daemon object for the purpose of tests.