Function schema_ref_reason 

pub fn schema_ref_reason(reference: &str, site: &str) -> String

The reason a schema $ref at site gives no name.

ref_target_name answers None for two different faults, and the remedy differs. A cross-file ref names a schema and still fails, so a message that asks the author to reference a schema misleads. Name the fault instead.

site reads into the sentence, for example a property.