improve error message when image save platform is not available

[knQzx]

summary

• image save now fails fast with a clear error when the requested platform is not available locally
• the message lists the platforms that ARE available so the user knows what they can save

before:
Error: notFound: "Content with digest sha256:..."

after:
Error: image alpine:latest has no content for platform linux/amd64; available platforms: linux/arm64

test plan

• verified locally against a multi-arch image with platforms amd64/arm64/ppc64le/s390x/386/arm-v6/arm-v7:
  • save --platform linux/arm/v5 (not in image) fails with the new error listing all real platforms (attestation unknown/unknown entries are filtered out)
  • save --platform linux/arm64 (in image) succeeds as before
  • save without --platform still saves the full multi-arch archive (pre-check is skipped)

fixes #874

[knQzx]

@jglogan added test

[jglogan]

@knQzx Thank you!