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.

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