Better error message for unicode formula in PY2

Closes pydata#56

patsy/highlevel.py

@@ -56,11 +56,8 @@ def _try_incr_builders(formula_like, data_iter_maker, eval_env,
                                       data_iter_maker,
                                       eval_env,
                                       NA_action)
-    else:
-        # check if formula_like is another string type (which might be unsupported)
-        if isinstance(formula_like, string_types):
-            raise PatsyError("Unsupported string type for formula (e.g., unicode in Python 2.X).")
-        return None
+    if not six.PY3 and isinstance(formula_like, unicode):
+        raise PatsyError("Unicode formula not support in Python 2")
 
 def incr_dbuilder(formula_like, data_iter_maker, eval_env=0, NA_action="drop"):
     """Construct a design matrix builder incrementally from a large data set.

patsy/test_highlevel.py

@@ -5,6 +5,7 @@
 # Exhaustive end-to-end tests of the top-level API.
 
 import sys
+import six
 import __future__
 import numpy as np
 from nose.tools import assert_raises
@@ -743,3 +744,11 @@ def test_C_and_pandas_categorical():
                            [[1, 0],
                             [1, 1],
                             [1, 0]])
+
+def test_unicode_py2():
+    if six.PY3:
+        return
+
+    data = {"x": [1, 2], "y": [1, 2]}
+    formula_like = u"y ~ x"
+    assert_raises(PatsyError, dmatrix, formula_like, data)