modules: bail out with proper message if Magnum is not found.
modules: bail out with proper message if Magnum is not found.

Currently it failed on "error when trying to open configure.h", which is
NOT a helpful message at all. Now we check for existence of the include
dir before trying to open the file, thus failing with proper "Could NOT
find Magnum" message.

DerThorsten/magnum,ashimidashajia/magnum,DerThorsten/magnum,ashimidashajia/magnum,MiUishadow/magnum,DerThorsten/magnum,ashimidashajia/magnum,DerThorsten/magnum,MiUishadow/magnum,DerThorsten/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum,ashimidashajia/magnum,ashimidashajia/magnum,MiUishadow/magnum