gcode: Raise a config error on invalid register_command()

Raise a printer.config_error() on an invalid register_command() call.
This error is easier to handle for the vast majority of callers.

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

klippy/extras/manual_probe.py

@@ -58,7 +58,7 @@ class ManualProbeHelper:
         try:
             self.gcode.register_command('ACCEPT', self.cmd_ACCEPT,
                                         desc=self.cmd_ACCEPT_help)
-        except self.gcode.error as e:
+        except self.printer.config_error as e:
             self.gcode.respond_error(
                 "Already in a manual Z probe. Use ABORT to abort it.")
             self.finalize_callback(None)