Throw in a human readable way if jsdom is not installed locally when running outside of a browser

Author: Munter

lib/index.js

@@ -433,7 +433,11 @@ module.exports = {
         htmlDocument.write(subject);
         htmlDocument.close();
       } else {
-        htmlDocument = require('jsdom').jsdom(subject);
+        try {
+          htmlDocument = require('jsdom').jsdom(subject);
+        } catch (err) {
+          throw new Error('The assertion `when parsed as html` was run outside a browser, but could not find the `jsdom` module. Please npm install jsdom to make this work.');
+        }
       }
       return this.shift(expect, htmlDocument, 0);
     });