hardcode error codes on metal

[bakkot]

Hopefully fixes #7009, though I haven't reproduced the issue myself so can't confirm.

[slaren]

Would it be possible to use the localizedDescription property instead? I imagine that would work everywhere. https://developer.apple.com/documentation/metal/mtlcommandbuffererror/2887326-localizeddescription

[bakkot]

Oh, sure, that's much nicer:

ggml_metal_graph_compute: command buffer 6 failed with status 5
error: Insufficient Memory (00000008:kIOGPUCommandBufferCallbackErrorOutOfMemory)

For some reason that's not documented on the Objective C docs (afaict), just the Swift ones, but accessing it in the obvious way does seem to work.