The ext impexp is mandatory to install a distribution
If you want to install a distibution, you must have the extension "impexp" (perhaps others?) installed. Otherwise a fatl error is thown.
A easy solution is that the option in the select box of the extension manager must not show the link to the preconfigured distribution if this extension is not installed. But it's not safe because it's possible to call the right URL directly.
An other solution, must safer, is to ask the user to install the extension before seeing the distribution choices.