[libomptarget] Harmonize emitting CUDA errors and general debug messages.
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
Suggested-by: Ye Luo <email@example.com>
Differential Revision: https://reviews.llvm.org/D65687