add error handling and message for when a browser is lacking

[jflec415git]

webapps would briefly freeze if the user sought to edit a webapp when its supported browser was no longer installed.

in on_edit_button

• updated the list[0] syntax to a next() statement to get the selected_browser_index
• added if block to handle None occurrence due to a missing browser

show_browser_not_found_dialog

• new method to handle dialog
• puts up a simple message that the browser is missing. exact text below:
  "The browser associated with this webapp no longer exists on your system. Please reinstall to edit or use this app."