Improve GCodeCommands

Summary:
From the old Diff

• Use Teareny opperation to make code more readable. Simplify Error Message

New in this Diff

• Rename GCodeCommands::toString -> GCodeCommands::description.
• Improve Error Message for when a Gcode is needed.

Reviewers: patrickelectric, laysrodrigues, tcanabrava

Reviewed By: patrickelectric, tcanabrava

Differential Revision: https://phabricator.kde.org/D10436