Ensures that CUDA fail reasons (such as "No CUDA-capable device detected") are printed together with libomptarget's debug message (e.g. "Error when setting CUDA context"). Previously, the former was printed only in CMAKE_BUILD_TYPE=Debug builds while the latter was enabled by LIBOMPTARGET_ENABLE_DEBUG,
With this change, also only call cuGetErrorString when the error will be printed.
Suggested-by: Ye Luo