silx view: Stopped displaying a message box for each error

[t20100]

This PR makes silx view using a dedicated sys.excepthook to stop showing a QMessageBox for each exception.
Instead, it keeps logging the error and adds a "permanent" widget in the application status bar.

I hesitated with a temporary error message, but since the effects of an exception might make the application unstable afterwards, I decided to use a permanent message instead.

closes #3880

[vallsv]

You better to use a system icon for the icon.

Maybe if you replace the QLabel by a QPushButton, or a QToolButton. Or just create a small panel.

Maybe a button is anyway better, because you could reset the error if you click, and display it in a message box (to allow to copy paste it)

BTW not totally related, but i have logging window in Flint, maybe that something i could backport here.

[vallsv]

What i have used in flint instead of a QLabel is a QToolBar+ a single QAction (maybe not the best idea).

And here is the stuff i use to display the message in flint

    def __doubleClicked(self, index):
        record = self.__logWidget.recordFromIndex(index)
        if record is None:
            return

        # _logger.error("%s %s %s", type_, value, ''.join(traceback.format_tb(trace)))
        msg = qt.QMessageBox()
        msg.setWindowTitle("Logging record")

        if record.levelno > logging.WARNING:
            icon = qt.QMessageBox.Critical
        elif record.levelno > logging.INFO:
            icon = qt.QMessageBox.Warning
        else:
            icon = qt.QMessageBox.Information
        msg.setIcon(icon)

        try:
            message = record.getMessage()
        except Exception as e:
            # In case there is a wrong call of logging methods
            message = "Error in logs: " + e.args[0]
            message += "\nMessage: %r" % record.msg
            message += "\nArguments: %s" % record.args

        cuts = message.split("\n", 1)

        msg.setInformativeText(cuts[0])
        msg.setDetailedText(message)
        msg.raise_()
        msg.exec()

[t20100]

Having a widget to display logging info would be nice in silx, if you find the time to move it here!

I reworked the way to display the error: It is now a toolbutton in the right corner of the menu bar which show a message box with the last exception when clicked.

Ready for review.