[MRG + 1] Clarified error msg in plot_partial_dependence (scikit-learn#7673)

* Clarified error msg in plot_partial_dependence

* Changed err msg for feature[i] out of range. Updated docs.

* Error message shows invalid value.

Author: kgilliam125

sklearn/ensemble/partial_dependence.py

@@ -183,10 +183,14 @@ def plot_partial_dependence(gbrt, X, features, feature_names=None,
         A fitted gradient boosting model.
     X : array-like, shape=(n_samples, n_features)
         The data on which ``gbrt`` was trained.
-    features : seq of tuples or ints
+    features : seq of ints, strings, or tuples of ints or strings
         If seq[i] is an int or a tuple with one int value, a one-way
         PDP is created; if seq[i] is a tuple of two ints, a two-way
         PDP is created.
+        If feature_names is specified and seq[i] is an int, seq[i]
+        must be < len(feature_names).
+        If seq[i] is a string, feature_names must be specified, and
+        seq[i] must be in feature_names.
     feature_names : seq of str
         Name of each feature; feature_names[i] holds
         the name of the feature with index i.
@@ -306,8 +310,9 @@ def convert_feature(fx):
                 l.append(feature_names[i])
             names.append(l)
     except IndexError:
-        raise ValueError('features[i] must be in [0, n_features) '
-                         'but was %d' % i)
+        raise ValueError('All entries of features must be less than '
+                         'len(feature_names) = {0}, got {1}.'
+                         .format(len(feature_names), i))
 
     # compute PD functions
     pd_result = Parallel(n_jobs=n_jobs, verbose=verbose)(