Open error dialog if user tries to use not-installed xdg-open - #1201

Thank you for working on this.

It might be better to pass self to gui_open via an optional gui? parameter, and open the dialog there. For osx and linux it could check for open or xdg-open, perhaps by storing the open or xdg-open in a variable and set it to None for windows. If the command is found or the command is None (windows), it could use gui.show_message_details() and display the exception in the details box to handle other errors. That way it works for all platforms, handles errors besides command not found, and works for the other gui_open call in on_open_download_folder.

However, the other call isn't trying to play a file, and would need a different error message. Unless gui_open doesn't check if the program exists and simply calls show_message_details.

Thank you for fixing this.