Univention Corporate Server 4.3 erratum 210 (bugfix)

This update addresses the following issues:
* Network errors while trying to contact the update server do not result in
  a traceback anymore. Instead, a readable error message is shown.
* Code for the deprecated UCS-2.x and UCS-3.x repository layout has been
  removed.
* The progress of a release update is written to a status file so that the
  package univention-maintenance-mode may read it to update the progress
  accordingly.
* Show a warning and allow examination of the updater.log file in UMC if
  the last release update failed.