Univention Corporate Server 4.0 erratum 203 (bugfix)

This update addresses the following issues:
* If the initialization of the App Center module fails an error message
  is shown.
* Display error message if contacting the LDAP server fails.
* The error messages when contacting the App Center server or the UCS
  activation fails have been improved.
* Errors when opening apt Packages files are now handled.
* Errors when the system has too few free disk space are now handled.
* Error messages which are raised from the package manager are now
  displayed instead of displaying a traceback.
* The wording for app maintainer has been adjusted to app provider.