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.

Allow HTML tags in external markdown files

Use textContent instead of innerHTML to prevent encoding of certain characters in Markdown code blocks.

update syntax highlight after editing (#210), move markdown and highlight scripts from lib to plugin