Display error toasts for screensharing, improve logs a bit more

...uebutton-html5/imports/api/audio/client/bridge/kurento.js

...on-html5/imports/api/screenshare/client/bridge/kurento.js

...rts/ui/components/actions-bar/desktop-share/component.jsx

bigbluebutton-html5/private/locales/en.json

...luebutton-html5/public/compatibility/kurento-extension.js