[MRG] 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`. I've pulled these out into a separate section, but I'm not sure that the formatting is appropriate (see screen shot below), or if there's a better way of presenting in Sphinx what might be `\paragraph` in LaTeX.

Apart from formatting, this PR is very minor.

![3 2 grid search searching for estimator parameters scikit-learn 0 16-git documentation 3](https://cloud.githubusercontent.com/assets/78827/5196376/50879dac-7582-11e4-9d2c-105ef62087f7.png)

if you move content, the matching references should follow.