Status Codes: mark not found resources as 404 (#1043)

And mark "unknown command" as 501 ("not implemented")

webdriver-spec.html

@@ -1608,7 +1608,7 @@ <h3>Handling Errors</h3>
 
  <tr>
   <td><dfn>no such alert</dfn>
-  <td>400
+  <td>404
   <td><code>no such alert</code>
   <td>An attempt was made to operate on a modal dialog
    when one was not open.
@@ -1633,15 +1633,15 @@ <h3>Handling Errors</h3>
 
  <tr>
   <td><dfn>no such frame</dfn>
-  <td>400
+  <td>404
   <td><code>no such frame</code>
   <td>A <a>command</a> to switch to a frame
    could not be satisfied because the frame could not be found.
  </tr>
 
  <tr>
   <td><dfn>no such window</dfn>
-  <td>400
+  <td>404
   <td><code>no such window</code>
   <td>A <a>command</a> to switch to a window
    could not be satisfied because the window could not be found.
@@ -1663,7 +1663,7 @@ <h3>Handling Errors</h3>
 
  <tr>
   <td><dfn>stale element reference</dfn>
-  <td>400
+  <td>404
   <td><code>stale element reference</code>
   <td>A <a>command</a> failed because
    the referenced <a>element</a> is no longer attached to the DOM.
@@ -1699,7 +1699,7 @@ <h3>Handling Errors</h3>
 
  <tr>
   <td><dfn>unknown command</dfn>
-  <td>404
+  <td>501
   <td><code>unknown command</code>
   <td>A <a>command</a> could not be executed
    because the <a>remote end</a> is not aware of it.