Misleading information when failed to load libcuda.

[takagi]

Fact

• When cl-cuda is being loaded, it is completed even if loading libcuda failed.
• After that, when foreign functions in libcuda are called, CL-CUDA.DRIVER-API::SDK-NOT-FOUND-ERROR is raised to show "CUDA SDK not found."

Problem

• We can not find out that the reason of the condition raised is CUDA SDK is not found or found but its loading has some problems.

See also
#42