Output error message when a command is not found. Closes #778

Plugins with commands will need to implement a catch-all command that exits with the `DOKKU_NOT_IMPLEMENTED_EXIT`` code (10). This signals to dokku that a given plugin has indeed not executed anything for a plugin (which may not always be the case).

Using plugins that do not implement this pattern will result in those plugins silencing the error message.
Merge pull request #780 from progrium/778-better-error-message

Output error message when a command is not found. Closes #778