gcode: Use the same underlying error object for homing and gcode errors

Introduce a homing.CommandError and use that as the basis for both
gcode and EndstopError exceptions.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/homing.py

@@ -146,7 +146,10 @@ class Homing:
             self.toolhead.motor_off()
             raise
 
-class EndstopError(Exception):
+class CommandError(Exception):
+    pass
+
+class EndstopError(CommandError):
     pass
 
 def EndstopMoveError(pos, msg="Move out of range"):