Add validation error detail in 'FenError'

src/board/chess_board/mod.rs

@@ -485,8 +485,8 @@ impl FromFen for ChessBoard {
             side,
         };
 
-        if !res.is_valid() {
-            return Err(FenError::InvalidPosition);
+        if let Err(err) = res.validate() {
+            return Err(FenError::InvalidPosition(err));
         }
 
         Ok(res)

src/fen.rs

@@ -1,4 +1,4 @@
-use crate::board::{CastleRights, Color, File, Piece, Rank, Square};
+use crate::board::{CastleRights, Color, File, InvalidError, Piece, Rank, Square};
 
 /// A trait to mark items that can be converted from a FEN input.
 pub trait FromFen: Sized {
@@ -13,16 +13,15 @@ pub enum FenError {
     /// Invalid FEN input.
     InvalidFen,
     /// Invalid chess position.
-    InvalidPosition,
+    InvalidPosition(InvalidError),
 }
 
 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)
+        match self {
+            Self::InvalidFen => write!(f, "Invalid FEN input"),
+            Self::InvalidPosition(err) => write!(f, "Invalid chess position: {}", err),
+        }
     }
 }