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.
modules: no backticks in CMake documentation.

That is only for Doxygen. Also improved the wording a bit.