#476 native head: report the cudaHostAlloc error, not just "cannot pin N MiB"

Beschreibung

The bare message cost a full debugging session: on Linux the failure was a missing libcuda.so.1 (a host driver library the nix loader cannot see) in a launch environment, not memory pressure.  cudaGetErrorString names the real cause, e.g. "CUDA driver version is insufficient for CUDA runtime version".