When the user specifies any of the distro, arch, or image type values to
filter generation, invalid combinations would cause a panic, which made
it hard to filter requests based just on an image type.
Instead of failing, print an error message to inform the user, but
continue with the rest of the jobs.
This way, a user is informed that a certain combination is invalid if
they make a mistake, but can also filter on a single image type and only
get valid manifests out of the run.
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