Better error reporting for external Markdown files

Better error reporting for external Markdown files

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.