`machines creates` complain about invalid architecture while it's in fact missing an image

When the `machines create` fails, here's what I get:

# maas admin boot-resources read
Success.
Machine-readable output follows:
[
    {
        "id": 1,
        "type": "Synced",
        "name": "ubuntu/focal",
        "architecture": "amd64/ga-20.04",
        "resource_uri": "/MAAS/api/2.0/boot-resources/1/",
        "subarches": "generic,hwe-p,hwe-q,hwe-r,hwe-s,hwe-t,hwe-u,hwe-v,hwe-w,ga-16.04,ga-16.10,ga-17.04,ga-17.10,ga-18.04,ga-18.10,ga-19.04,ga-19.10,ga-20.04"
    },
    {
        "id": 2,
        "type": "Synced",
        "name": "ubuntu/focal",
        "architecture": "amd64/ga-20.04-lowlatency",
        "resource_uri": "/MAAS/api/2.0/boot-resources/2/",
        "subarches": "generic,hwe-p,hwe-q,hwe-r,hwe-s,hwe-t,hwe-u,hwe-v,hwe-w,ga-16.04,ga-16.10,ga-17.04,ga-17.10,ga-18.04,ga-18.10,ga-19.04,ga-19.10,ga-20.04"
    },
    {
        "id": 3,
        "type": "Synced",
        "name": "ubuntu/focal",
        "architecture": "amd64/hwe-20.04",
        "resource_uri": "/MAAS/api/2.0/boot-resources/3/",
        "subarches": "generic,hwe-p,hwe-q,hwe-r,hwe-s,hwe-t,hwe-u,hwe-v,hwe-w,ga-16.04,ga-16.10,ga-17.04,ga-17.10,ga-18.04,ga-18.10,ga-19.04,ga-19.10,ga-20.04"
    },
    {
        "id": 4,
        "type": "Synced",
        "name": "ubuntu/focal",
        "architecture": "amd64/hwe-20.04-edge",
        "resource_uri": "/MAAS/api/2.0/boot-resources/4/",
        "subarches": "generic,hwe-p,hwe-q,hwe-r,hwe-s,hwe-t,hwe-u,hwe-v,hwe-w,ga-16.04,ga-16.10,ga-17.04,ga-17.10,ga-18.04,hwe-18.10,hwe-19.04,hwe-19.10,hwe-20.04"
    },
    {
        "id": 5,
        "type": "Synced",
        "name": "ubuntu/focal",
        "architecture": "amd64/hwe-20.04-lowlatency",
        "resource_uri": "/MAAS/api/2.0/boot-resources/5/",
        "subarches": "generic,hwe-p,hwe-q,hwe-r,hwe-s,hwe-t,hwe-u,hwe-v,hwe-w,ga-16.04,ga-16.10,ga-17.04,ga-17.10,ga-18.04,ga-18.10,ga-19.04,ga-19.10,ga-20.04"
    },
    {
        "id": 6,
        "type": "Synced",
        "name": "ubuntu/focal",
        "architecture": "amd64/hwe-20.04-lowlatency-edge",
        "resource_uri": "/MAAS/api/2.0/boot-resources/6/",
        "subarches": "generic,hwe-p,hwe-q,hwe-r,hwe-s,hwe-t,hwe-u,hwe-v,hwe-w,ga-16.04,ga-16.10,ga-17.04,ga-17.10,ga-18.04,hwe-18.10,hwe-19.04,hwe-19.10,hwe-20.04"
    }
]

As mentioned in the original report, this is due to images not being synced yet. We should probably error out earlier when images haven't been synced with a message specifying such. Marking as triaged.