Error actions use HTTP code name for title

Change the title of error actions to the HTTP code name, like
'internal server error'.

lib/clienterroraction.php

@@ -91,4 +91,9 @@ class ClientErrorAction extends ErrorAction
 
         $this->showPage();
     }
+
+    function title()
+    {
+        return $this->status[$this->code];
+    }
 }

lib/servererroraction.php

@@ -89,4 +89,8 @@ class ServerErrorAction extends ErrorAction
         $this->showPage();
     }
 
+    function title()
+    {
+        return $this->status[$this->code];
+    }
 }