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.

plugin/markdown/markdown.js

@@ -101,12 +101,21 @@
 
                 xhr.onreadystatechange = function () {
                     if( xhr.readyState === 4 ) {
-                        section.outerHTML = slidifyMarkdown( xhr.responseText, section.getAttribute('data-separator'), section.getAttribute('data-vertical') );
+                        if (xhr.status >= 200 && xhr.status < 300) {
+                            section.outerHTML = slidifyMarkdown( xhr.responseText, section.getAttribute('data-separator'), section.getAttribute('data-vertical') );
+                        } else {
+                            section.outerHTML = '<section data-state="alert">ERROR: The attempt to fetch ' + url + ' failed with the HTTP status ' + xhr.status +
+                                '. Check your browser\'s JavaScript console for more details.</section>';
+                        }
                     }
                 };
 
                 xhr.open('GET', url, false);
-                xhr.send();
+                try {
+                    xhr.send();
+                } catch (e) {
+                    alert('Failed to get the Markdown file ' + url + '. Make sure that the presentation and the file are served by a HTTP server and the file can be found there. ' + e);
+                }
 
             } else if( section.getAttribute('data-separator') ) {