Validate snapd payload

[sergiusens]

What needs to get done

We need to validate the snapd API return payload in craft_parts/packages/snaps.py

Why it needs to get done

This will allow us to gracefully handle errors such as

  File "/snap/snapcraft/12504/lib/python3.10/site-packages/snapcraft_legacy/internal/repo/snaps.py", line 295, in install_snaps
    snap_pkg_channel = snap_info["channel"]
TypeError: 'NoneType' object is not subscriptable