[MRG+2] DOC narrative docs for grid search's robustness to failure

A fairly minor doc and test improvement. The `error_score` parameter in grid search wasn't mentioned in narrative docs.

I realised there were a number of best practice tips associated with parameter search, not specific to `GridSearchCV` or `RandomizedSearchCV`.

![3 2 grid search searching for estimator parameters scikit-learn 0 16-git documentation 4](https://cloud.githubusercontent.com/assets/78827/5199242/315ca3a6-75ae-11e4-9f99-212d6b0bbabf.png)