Reduce verbosity of error messages, with addition of 500 and 429 pages

- Reduce verbosity of error messages in existing error pages with a
  short description of the error which occurred, followed by a bullet
  point list of probable causes.
- Addition of HTTP Error 500 and 429 pages.
- Add .gitignore file

input/error/404.html

@@ -4,4 +4,11 @@
 <div class="err">
     <h1>Not Found</h1>
     <p>The resource requested not found on this server.</p>
+
+    <p>There could be a few reasons for this:</p>
+
+    <ul>
+        <li>The resource requested is no longer available, or;</li>
+        <li>There might be an error in the address provided.</li>
+    </ul>
 </div>