small fix (pymc-devs#1284)

This construction could accidentally fail

Author: ferrine

pymc3/distributions/distribution.py

@@ -27,7 +27,7 @@ def __new__(cls, name, *args, **kwargs):
         elif name is None:
             return object.__new__(cls)  # for pickle
         else:
-            raise TypeError("needed name or None but got: " + name)
+            raise TypeError("needed name or None but got: %s" % name)
 
     def __getnewargs__(self):
         return None,