Don't throw on unsupported heatmap layer type instead show helpful debug info

[orangemug]

Previous to this change a style with a heatmap layer type would "throw" crashing the app. We now just console.debug with

[ol-mapbox-style] layers[1].type "heatmap" not supported

Reasons for console.debug instead of console warn/log, is that console.debug doesn't show up in the console by default. You have to enable verbose logging in the web inspector.

This also has the same behaviour as other unsupported styling rules, for example icon-pitch-alignment

[ahocevar]

I'd prefer console.error over console.debug. It is easier to see when something goes wrong for those who have a developer console open, and others won't bother anyway.

[orangemug]

The only problem with console.error is it's very noisy. So if you load a style that's has a heatmap, it is a valid style but it'll complain a lot. That means that if a person uses this library in their map (like I do) I get spammed with errors I have no way of currently resolving. For me this is on every style change as I update styles a lot when editing.

Also it's a little odd that layer types error when unsupported but unsupported properties don't.

Other options, worth considering

1. Just don't log anything, which is in keeping with the rest of the libraries properties (later we could add a function to report/remove the unsupported properties, once Patched Mapbox GL Spec #10 is complete)
2. Log via debug which isn't noisy by default, would require prod / non-prod builds (probs too complex for now)
3. @orangemug just go ahead and use console.error(..)

Please pick an option (1, 2, or 3)

[ahocevar]

For the case of things that are not supported by ol-mapbox-style, I think you're right that a debug message is appropriate.

[orangemug]

I've made the changes, hopefully this should be good to merge now.