luxmark: catch OpenCL errors like CL_DEVICE_NOT_FOUND when listing devices

[illwieckz]

The application should not crash because there is no OpenCL device.

This avoids crashing on:

terminate called after throwing an instance of 'std::runtime_error'
  what():  OpenCL driver API error (code: -1, file:LuxCore/src/luxrays/devices/ocldevice.cpp, line: 89): CL_DEVICE_NOT_FOUND

This work is courtesy of I ♥ Compute initiative funded by 🇫🇷️ rebatir.fr.