mark Unknown variant as hidden

to discourage explicitly matching this variant explicitly.
As the enum is non_exhaustive a wildcard pattern is required anyway.

Author: Skgland

src/prepare.rs

@@ -391,6 +391,7 @@ pub enum PrepareError {
     #[error("the crate depends on missing dependencies: \n\n{0}")]
     MissingDependencies(String),
     /// Some not further detaild error
+    #[doc(hidden)]
     #[error("prepare failed without further details")]
     Unknown,
 }