MAINT, DOC: Modify Hyperband docs for first-time users

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What does this PR implement?
@ToddMorrill has a good example of Hyperband in dask/dask-examples#149. He's a first time user and has hit some road bumps.

This PR does the following:

• More clearly showed errors while scoring models (as in BUG: clearly show error message while initing models #670).
• Better names variables in HyperbandSearchCV.metadata (I'm open to suggestions!)
• Adds some notes to Hyperband's documentation.
• Warns the user about unbalanced chunks.
• Adds a comparison with RandomizedSearchCV to HyperbandSearchCV.metadata.

Reference issues/PRs

• BUG: clearly show error message while initing models #670.
• This example is based off of initial skorch hyperparam opt implementation dask-examples#149.

TODO:

• change n_models to n_params, and add note in documentation about it being approximate
• (blog posts) change rule of thumb to evenly chunk dask array.

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• make clear that each partial_fit call sees one chunk of the data