[MRG] FIX: Raise error when n_estimators = 0 in ensemble models.

It is possible to do:
`clf = ensemble.RandomForestClassifier(n_estimators=0)`
then 
`clf.fit(X, y)` raises a division by zero error.

0.14.1 raised a ValueError, behavior changed with PR: 
ENH bagging meta-estimator.
74d395215bbd2009dc922967b41c43f5469942ce

This PR just resets the 0.14.1 behavior.
This type of error message are even better if you make something like
```
raise ValueError("n_estimators must be greater than zero, got {1}.".format(self.n_estimators))
```