enabling errors for gpu-disabled devices

[dkazanc]

Fixes issue

Checklist

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have made corresponding changes to the documentation
• I have added the user-release-note label in order to include this PR in the "Notable
  Changes for Users" section in release notes

[dkazanc]

@yousefmoazzam I managed to get to a device without GPU to test this. I discovered that it wasn't actually possible to run CPU-only pipelines due to a couple of conditions that are fixed now in this PR. I then also discovered that by this condition in wrappers we usually pick up earlier if the pipeline contains GPU methods when running on the CPU-only device:

        if self.is_gpu and not gpu_enabled:
            raise ValueError("GPU is not available, please use only CPU methods") 

So I removed my changes on checking if pipeline is a GPU or not as it comes AFTER the pipeline generation step in CLI.
And I moved the paganin method import in the sweep runner to avoid the issue we discussed.

[yousefmoazzam]

Looks good to me, only one suggestion, feel free to take it or leave it!

Since the iris CI has been pretty bad lately, I'll also just note to make sure tests pass locally.