GRBL Features and a bug fix

- Implemented the % command for wiping the SPJS buffer - % does not get passed on to GRBL as it is an unrecognized command and will return an error.

- Added a return type of Error (in lieu of 'Complete') for when a gcode line returns an error
from GRBL - Hope to further enhance how chilipeppr responds when an error is detected.

- Fixed bug causing buffer to halt all sending to GRBL.