policies: improve error response

[migmartri]

When a remote policy fails to be fetched we show an error that doesn't tell me which one actually failed.

We should show which one couldn't be loaded.

chainloop att add --name sbom --value ./sbom.cyclonedx.json
INF uploading sbom.cyclonedx.json - sha256:16159bb881eb4ab7eb5d8afc5350b0feeed1e31c0a268e355e74f9ccbe885e0c
ERR adding material: error applying policies to material: policy error: failed to load policy spec: loading policy spec: loading policy: rpc error: code = NotFound desc = policy not found

On another note, and maybe as part of #1207, we should also figure out how to extract the basic error from the grpc.

btw, this is the contract I used

{
  "schemaVersion": "v1",
  "materials": [
    {
      "type": "SBOM_CYCLONEDX_JSON",
      "name": "sbom"
    }
  ],
  "policies": {
    "materials": [
      {
        "ref": "cyclonedx-licenses.yaml"
      },
      {
        "ref": "https://raw.githubusercontent.com/chainloop-dev/chainloop/main/docs/examples/policies/sbom/cyclonedx-licenses.yaml"
      },
      {
        "ref": "chainloop://cyclonedx-licenses"
      },
      {
        "ref": "chainloop://platform/cyclonedx-licenses"
      }
    ]
  }
}