Surface better error messages in crane index

[jonjohnsonjr]

If we're trying to do things to a non-index, we should fail earlier.

Fixes: #1720

Before:

$ crane index filter docker.io/neuvector/scanner:latest --platform linux/amd64
Error: pulling docker.io/neuvector/scanner:latest: unsupported MediaType: "application/vnd.docker.distribution.manifest.v1+prettyjws", see https://github.com/google/go-containerregistry/issues/377

After:

$ crane index filter docker.io/neuvector/scanner:latest --platform linux/amd64
Error: expected docker.io/neuvector/scanner:latest to be index, got "application/vnd.docker.distribution.manifest.v2+json"