Submitting mods without valid versions should warn/error #34

Platform filters project versions to only include those valid for the version of the event. Thus, it can easily happen that a version isn't properly marked and there's no valid version for a project. Right now, that means the version is silently set to null which causes the project to be ignored. This is not what should happen, submitting a project without a valid version should error.

Note that for blanketcons people might submit modpacks. These ought to be exempt, as it's intended for them not to be added to the pack. Also note we might intentionally want to set the version to null for reasons™.

Relevant code: