metal : print error of load pipeline state

[jhen0409]

Tracking #2550.

This change add NSError to newComputePipelineStateWithFunction call so we can easier to see the error. The following error is from A12Z iPad Pro:

ggml_metal_init: load pipeline error: Error Domain=AGXMetalA12 Code=3 "Encountered unlowered function call to air.simd_sum.f32" UserInfo={NSLocalizedDescription=Encountered unlowered function call to air.simd_sum.f32}