#1163356 by jonathan1055, salvis: Add error handler option to display backtrace as well as standard message.

(cherry picked from commit 6286aff5)

Avoid "Notice: Undefined property: stdClass::$timestamp in _drupal_session_write()" watchdog entry when switching users.

Remove unused devel/source page callback. Theme developer can implement its own if it wants to keep this feature.

'restrict access' => TRUE for various permissions. See http://drupal.org/update/modules/6/7#permissions_restrict_access

Suppress "Cannot modify header information - headers already sent" warning if we caused it ourselves.

#992938 by Dave Reid. Fix the switch user block should use format_username(), which requires full user objects.