Refactor Sudoku field and solver implementation

field/errors.go

@@ -3,6 +3,8 @@ package field
 import "errors"
 
 var (
-	outOfBound   = errors.New("invalid coordinates")
-	invalidField = errors.New("can not parse Field")
+	ErrOutOfBounds   = errors.New("coordinates out of bounds")
+	ErrInvalidField  = errors.New("invalid field")
+	ErrInvalidCell   = errors.New("invalid cell")
+	ErrInvalidChange = errors.New("invalid change")
 )