1. Show an alert when loading the file throws an exception
2. Whenever the status isn't success (2xx) - upon exception or e.g.
"file not found" - replace the section's content with information
about the failure (instead of being silent about it and showing
a blank slide).
We need to inform the user something went wrong, not just be silent
about it. An experienced developer, upon seing a blank slide, will
likely check the javascript console - but the rest will profit from
being told about the problem (and solution) explicitely.
