Fix build of binary package

Commit a00437 accidentally changed that the source-package error is also
printed for binary packages. Because it defined
CPACK_SOURCE_PACKAGE_FILE_NAME which we used to detect source-package.
This now uses CPACK_INSTALL_CMAKE_PROJECTS which is hopefully future-proof.
It also adds a proper warning for binary packages.

Change-Id: I842655755bed1c448f30db435a85ad1ca5844a60