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/500.html

@@ -0,0 +1,16 @@
+[//]: # "Title: Internal Server Error"
+[//]: # "Summary: An internal server error has occurred."
+
+<div class="err">
+    <h1>Internal Server Error</h1>
+    <p>An internal server error has occurred.</p>
+
+    <p>There could be a few reasons for this:</p>
+
+    <ul>
+        <li>An internal misconfiguration of this resource, or;</li>
+        <li>Maintenance of this resource is occurring.</li>
+    </ul>
+
+    <p>Please try again later, or <a href="https://status.sapphirus.net">check here</a> for further information.</p>
+</div>