[imp] show disabled update sites warning when searching for extensionupdates

see: https://github.com/joomla/joomla-cms/commit/995d26c91f6208ba4d1d9869323c31ebcd986187
Update de-DE.com_installer.ini
Merge pull request #73 from zero-24/patch-32

[imp] show disabled update sites warning when searching for extensionupdates