ReferenceOr<Schema> should probably return a result

[Troels51]

I switched to using your fork for an experiment because it already has "resolve" as a function on the References.
Most of them return a Result except for the one on ReferenceOr<Schema>, and it seems that it should too

openapiv3/src/reference.rs

Line 156 in cb7946b

pub fn resolve<'a>(&'a self, spec: &'a OpenAPI) -> &'a Schema {

vs

openapiv3/src/reference.rs

Line 203 in cb7946b

pub fn resolve<'a>(&'a self, spec: &'a OpenAPI) -> Result<&'a Parameter> {