Changed up/down behavior on errors.

* Errors are not reported via the exit status.
* If actions are forced with '-f', interface configuration and
  defconfiguration commands will be executed even if there were previous
  errors.
* If an interface has an error being brought up and is not being forced,
  it will be put down in order to prevent leaving it in an unstable
  state
* Functions should return true on success or if being forced, and only
  return false if failure and not being forced.
* This basically means that the script will always execute with success
  (return value 0) when being forced