Tests: Add diagnostic to ExportImport test.

The error message is otherwise confusing:

 $ make
 makeobj[0]: Entering directory `/home/stephen/dev/src/cmake/Tests/ExportImport/build'
 [ 25%] Generating CleanupProject
 [ 25%] Built target CleanupTarget
 [ 50%] Generating ExportProject
 loading initial cache file /home/stephen/dev/src/cmake/Tests/ExportImport/build/InitialCache.cmake
 Internal cmake changing into directory: /home/stephen/dev/src/cmake/Tests/ExportImport/build/Export
 Error: cmake execution failed
 CMake Error: CMake was unable to find a build program corresponding to "Unix Makefiles".  CMAKE_MAKE_PROGRAM is not set.  You probably need to select a different build tool.
 CMake Error: Error required internal CMake variable not set, cmake may be not be built correctly.
Merge topic 'minor-cleanups' into next

2a66d7e Tests: Add diagnostic to ExportImport test.