Add a shapfile without the prj file

[silviafranceschi]

I tried to add a shapefile from the layers list, this shapefile didn't have the prj file. The application starts to elaborate the data but the shapefile will be not displayed and not error messages are shown. Maybe it would be useful to add two possibilities:

1. once the data has no prj star asking the projection to the user
2. display an error message requiring the missing information.