[MRG+1] Clarify error message for min_samples_split. - #8167

~ $ cat example.py 
from sklearn.tree import DecisionTreeClassifier
dtc = DecisionTreeClassifier(min_samples_split=1)
dtc.fit([[0]], [0])
~ $ python example.py 
Traceback (most recent call last):
  File "example.py", line 3, in <module>
    dtc.fit([[0]], [0])
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/sklearn/tree/tree.py", line 739, in fit
    X_idx_sorted=X_idx_sorted)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/sklearn/tree/tree.py", line 199, in fit
    % self.min_samples_split)
ValueError: min_samples_split must be at least 2 or in (0, 1.0], got 1