errors: change timeout errors from 408 to 500 HTTP status code

Whilst it is logically correct to use 408 for the timeout and script
timeout errors, it causes a collision with HTTP semantics implement
in HTTP clients.  To support Keep-Alive we allow retries in HTTP
clients and if a client sees code 408 it thinks that the server has
gone away and retries the request.

This causes Execute Script, Navigate To, and Refresh commands to
be sent twice with some HTTP clients.

This is a backwards incompatible change to WebDriver in order to
not break HTTP/1.1 Keep-Alive.

Fixes: #1287

index.html

@@ -1728,7 +1728,7 @@ <h3>Handling Errors</h3>
 
  <tr>
   <td><dfn>script timeout</dfn>
-  <td>408
+  <td>500
   <td><code>script timeout</code>
   <td>A script did not complete before its timeout expired.
  </tr>
@@ -1750,7 +1750,7 @@ <h3>Handling Errors</h3>
 
  <tr>
   <td><dfn>timeout</dfn>
-  <td>408
+  <td>500
   <td><code>timeout</code>
   <td>An operation did not complete before its timeout expired.
  </tr>