Make platform errors less cryptic

[AkihiroSuda]

Fix #36

New output:

$ diffoci diff alpine:3.18.2 alpine:3.18.3
INFO[0000] Target platforms: [darwin/amd64]
INFO[0000] Pulling "docker.io/library/alpine:3.18.2" for additional platforms ([darwin/amd64])
docker.io/library/alpine:3.18.2                 saved
└──index (82d1e9d7ed48)                         already exists
Pulling from OCI Registry (docker.io/library/alpine:3.18.2)     elapsed: 2.0 s  total:  1.6 Ki  (839.0 B/s)
INFO[0002] Pulling "docker.io/library/alpine:3.18.3" for additional platforms ([darwin/amd64])
docker.io/library/alpine:3.18.3                 saved
└──index (7144f7bab3d4)                         already exists
Pulling from OCI Registry (docker.io/library/alpine:3.18.3)     elapsed: 0.9 s  total:  1.6 Ki  (1.7 KiB/s)
ERRO[0002] image 0 is not available for the requested platform: unavailable (Hint: specify `--platform` explicitly, e.g., `--platform=linux/amd64`)