debug messages for bad position data

src/position.rs

@@ -10,13 +10,13 @@ impl Position {
     pub(crate) fn from(string: String) -> Result<Position, &'static dyn Error> {
         // e.g. "2,5"
         let xy: Vec<&str> = string.split(',').collect();
-        let x = xy[0].parse().unwrap();
+        let x = xy[0].parse().expect("Bad x coordinate supplied for Position");
 
         if xy.len() < 2 {
             panic!("Bad position : {}", string);
         }
 
-        let y = xy[1].parse().unwrap();
+        let y = xy[1].parse().expect("Bad y coordinate supplied for Position");
 
         Ok(Position { x, y })
     }