Automatically infer the response type and produce helpful error

[ledell]

There's a classification button, which is set to classification by default. However, that makes it easy to train a classification model when you really want a regression model. If the response is real-valued and you try to do classification, it currently just breaks wave (e.g. wine quality data using "alcohol" as response). So we need a better error.

Another issue is when you have an integer valued column as the response which should be numeric/regression (e.g. wine quality using "quality" as response), it will still train a classification model when asked to. You don't realize that until after you read the column headers in the leaderboard.

[ledell]

We should do this on the backend instead: https://h2oai.atlassian.net/browse/PUBDEV-8402?filter=20700