fn validate_assets(m: &Manifest) -> Result<(), String>Expand description
Validate a manifest’s assets and the routine references to them. Only
content plugins may carry assets; each must be a sound, in-cap image or
audio file (see validate_asset) with a unique id. Every step.asset
must resolve to an image asset, and every step.sound / breath phase cue
to an audio asset. First-error-wins, like the rest of the file.