opencl backend: handle failure to open files more gracefully
opencl backend: handle failure to open files more gracefully

That is, print an error message instead of dumping core.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

nicolasvasilache/ppcg,Meinersbur/ppcg,nicolasvasilache/ppcg,Meinersbur/ppcg