qtvcp: don't pop a modal error dialog when headless

The excepthook shows a modal QMessageBox for uncaught exceptions. Headless
(offscreen/CI) nobody can dismiss it, so it blocks forever; if the error is
during construction it hangs before SIGTERM is armed and qtdragon-quit times
out. When offscreen (or no QApplication), log the traceback, run shutdown,
and exit instead. Interactive path unchanged.

Author: grandixximo

src/emc/usr_intf/qtvcp/qtvcp.py

@@ -556,6 +556,21 @@ def excepthook(self, exc_type, exc_obj, exc_tb):
                     + "information may be useful in troubleshooting:\n"
                     + 'LinuxCNC Version  : %s\n'% self.INFO.LINUXCNC_VERSION)
 
+            # Headless/offscreen (e.g. CI): nobody can dismiss a modal dialog,
+            # so msg.exec_() below would block forever. Report the error the
+            # normal way (stderr + log + non-zero exit) and quit instead of
+            # popping a dialog; the error is still visible, just not modal.
+            app = QtWidgets.QApplication.instance()
+            if app is None or app.platformName() == 'offscreen':
+                sys.stderr.write(''.join(lines))
+                sys.stderr.flush()
+                LOG.critical("Qtvcp error (headless):\n{}".format(''.join(lines)))
+                try:
+                    self.shutdown()
+                except Exception:
+                    pass
+                os._exit(1)
+
             msg = QtWidgets.QMessageBox()
             msg.setIcon(QtWidgets.QMessageBox.Critical)
             msg.setText(self._message)