homing: Prefer printer.command_error() instead of homing.CommandError()

Update callers to use the printer.command_error reference instead of
directly using homing.CommandError() when raising or catching errors.

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

klippy/stepper.py

@@ -4,7 +4,7 @@
 #
 # This file may be distributed under the terms of the GNU GPLv3 license.
 import math, logging, collections
-import homing, chelper
+import chelper
 
 class error(Exception):
     pass