Add 'FenError::InvalidPosition' variant

src/fen.rs

@@ -12,12 +12,15 @@ pub trait FromFen: Sized {
 pub enum FenError {
     /// Invalid FEN input.
     InvalidFen,
+    /// Invalid chess position.
+    InvalidPosition,
 }
 
 impl std::fmt::Display for FenError {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         let error_msg = match self {
             Self::InvalidFen => "Invalid FEN input",
+            Self::InvalidPosition => "Invalid chess position",
         };
         write!(f, "{}", error_msg)
     }