This PR makes some minor adjustments to desktop.notify with the aim of providing working fallback mechanisms for desktop environments that don't support traditional desktop notifications:

• On Linux-like systems, actually catch and handle exceptions raised by txbus calls in the event that, e.g., the host operating system lacks DBus (these exceptions were previously uncaught due to missing yield statement...)
• Show a popup message via QSystemTray.showMessage iff the systray actually supports messages
• Fall back to logging the message, if all else fails