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/429.html

@@ -0,0 +1,17 @@
+[//]: # "Title: Too Many Reqests"
+[//]: # "Summary: Access to this resource has been throttled."
+
+<div class="err">
+    <h1>Too Many Requests</h1>
+    <p>Access to this resource has been throttled.</p>
+
+    <p>There could be a few reasons for this:</p>
+
+    <ul>
+        <li>The rate limit for this resource has been exhausted, or;</li>
+        <li>An excessive amount of traffic to this resource is 
+            originating from malicious traffic on your network.</li>
+    </ul>
+
+    <p>Please try again later.</p>
+</div>