[llvm] [Offload] Ensure all `llvm::Error`s are handled (PR #137339)

[Ross Brunton via llvm-commits]

RossBrunton wrote:

> These definitely slip by, thanks for addressing it. Think we could have a utility function that converts an LLVM error into one of the offload ones? In the future ideally we will emit the LLVM errors with the appropriate codes and we can just forward the contained code.

I think a utility method makes sense, but that's probably something to do/decide on later once how error codes are going to be passed to offload is more fleshed out.

https://github.com/llvm/llvm-project/pull/137339