better error string

Author: darijgr

src/sage/combinat/sf/kfpoly.py

@@ -89,7 +89,7 @@ def KostkaFoulkesPolynomial(mu, nu, t=None):
     if mu not in _Partitions:
         if mu in SkewPartitions():
             return kfpoly_skew(mu, nu, t)
-        raise ValueError("mu must be a partition")
+        raise ValueError("mu must be a partition or skew partition")
     if nu not in _Partitions:
         raise ValueError("nu must be a partition")
     if sum(mu) != sum(nu):