`VaryingError::InvalidBlendSrcIndex` is too general

[Wumpf]

This diagnostic's quality should be refined. We give the entire set of criteria that need to be upheld here, but a succinct diagnostic really only need inform the user of the specific rule(s) they're violating and points them towards reference materials for the full set. Let's file an issue to follow up with this.

Originally posted by @ErichDonGubler in #7146 (comment)