Improve feedback to user after a failed file upload

...n-html5/imports/api/presentations/server/eventHandlers.js

...entations/server/handlers/presentationConversionUpdate.js

...mponents/presentation/presentation-uploader/component.jsx

...mponents/presentation/presentation-uploader/container.jsx

.../components/presentation/presentation-uploader/service.js

...components/presentation/presentation-uploader/styles.scss

bigbluebutton-html5/private/config/settings.yml

bigbluebutton-html5/public/locales/en.json