clarify type error for Column.substr()

Author: nchammas

python/pyspark/sql/column.py

@@ -406,7 +406,13 @@ def substr(self, startPos, length):
         [Row(col=u'Ali'), Row(col=u'Bob')]
         """
         if type(startPos) != type(length):
-            raise TypeError("Can not mix the type")
+            raise TypeError(
+                "startPos and length must be the same type. "
+                "Got {startPos_t} and {length_t}, respectively."
+                .format(
+                    startPos_t=type(startPos),
+                    length_t=type(length),
+                ))
         if isinstance(startPos, (int, long)):
             jc = self._jc.substr(startPos, length)
         elif isinstance(startPos, Column):