warn the user that the plugin cannot use raster charts

[wellenvogel]

When using the shop registration (or the offline process) the plugin has no control on which charts the user tries to assign. So a user can accidently buy and assign raster charts - but the plugin will later simply fail to load them.

So there should be some warning dialogs in the plugin UI about this problem. At least when downloading an fpr (for the offline process) and when registering at the shop or when routing to the shop page.
The warning should most probably only occur once on every system.

[wellenvogel]

There should also be a meaningful error when trying to upload raster charts.