GladeVCP + Python: error handling during G38.2

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import os
import linuxcnc
from datetime import datetime

class HandlerClass:

    # Prints message in following format: current time + text + LF code
    def msg(self, text):
        end_iter = self.buffer.get_end_iter()
        self.buffer.insert(end_iter, "%s %s\n" % (datetime.now().strftime('%H:%M:%S'), text))
        self.textview.scroll_to_mark(self.buffer.get_insert(), 0)

    # Returns -1 if LinuxCNC error detected
    def error_poll(self):
        error = self.e.poll()
        if error:
            kind, text = error
            if kind in (linuxcnc.NML_ERROR, linuxcnc.OPERATOR_ERROR):
                return -1
        return 0

    # Switching to MDI mode
    def set_mdi_mode(self):
        self.command.mode( linuxcnc.MODE_MDI )
        self.command.wait_complete()

    # Switching to manual mode
    def set_manual_mode(self):
        self.command.mode( linuxcnc.MODE_MANUAL )
        self.command.wait_complete()

    # Line by line G-Code execution
    def gcode(self, s):
        for s in s.split("\n"):
            self.command.mdi(s)
            self.command.wait_complete()
            if self.error_poll() == -1:
                return -1
        return 0

    # On button click
    def on_button_clicked(self, widget, data = None):
        self.msg("Button pressed...");

        # Set MDI mode
        self.set_mdi_mode()

        # A simple G-Code
        cmd = """G91
        G38.2 X-20 F300
        G90"""

        # Executing G-Code
        if self.gcode(cmd) == -1:
            self.msg("ERROR!")
        else:
            self.msg("OK!")

        # Set manual mode
        self.set_manual_mode()

# INIT FUNCTION
    def __init__(self, halcomp, builder, useropts):
        self.builder = builder
        self.stat = linuxcnc.stat()
        self.command = linuxcnc.command()
        self.e = linuxcnc.error_channel()
        self.textview = builder.get_object("textview")
        self.buffer = self.textview.get_property('buffer')
        # Welcome message
        end_iter = self.buffer.get_end_iter()
        self.buffer.insert(end_iter, "Welcome!\n\n")

# HANDLERS
def get_handlers(halcomp, builder, useropts):
    return [HandlerClass(halcomp, builder, useropts)]

...
    # Line by line G-Code execution
    def gcode(self, s):
        for l in s.split("\n"):
            self.command.mdi(l)
            # Default wait_complete() timeout is 5 seconds,
            # so 30 seconds should be enough for slow G38 operations
            self.command.wait_complete(30)
            #if self.error_poll() == -1:
            #    return -1
        return 0

    # On button click
    def on_button_clicked(self, widget, data = None):
        self.msg("Button pressed...");

        # Set MDI mode
        self.set_mdi_mode()

        # A simple G-Code
        cmd = """G91
        G38.2 X-20 F300
        G90"""

        # Executing G-Code
        self.gcode(cmd)

        # Check probe status
        self.stat.poll()
        if self.stat.probe_tripped == False:
            self.msg("ERROR!")
        else:
            self.msg("OK!")

        # Set manual mode
        self.set_manual_mode()
...