More helpful error message

[daladim]

This change is related to #1312 and #1377.

Every so often, I'm hitting a TypeTree must have a path error. Usually, it can be worked around by creating a newtype that somehow is better supported by utoipa (see an example here).
I'm not versed enough in proc-macros to fix this myself. However, I have been using this trick, that at least points me at what type is not "understood" by utoipa.

Example of error message before:

TypeTree must have a path

Example of error message after:

utoipa does not understand this type, for which type_tree must have a path:: Type::Tuple { paren_token: Paren, elems: [Type::Path { qself: None, path: Path { leading_colon: None, segments: [PathSegment { ident: Ident { ident: "SomeCustomType", span: #0 bytes(174539..174557) }, arguments: PathArguments::None }] } }, Comma, Type::Path { qself: None, path: Path { leading_colon: None, segments: [PathSegment { ident: Ident { ident: "T", span: #0 bytes(174559..174560) }, arguments: PathArguments::None }] } }] }