[node] Throw on invalid style JSON

[tmpsantos]

Fixes #6143

[mention-bot]

@tmpsantos, thanks for your PR! By analyzing this pull request, we identified @jfirebaugh, @mikemorris and @tmcw to be potential reviewers.

[mikemorris]

Hrm, it would be nice if map.load took a callback arg that it could pass an error to (I think it's actually async already despite the API design?) instead of needing to wrap this in a try/catch in implementing code...

That being said, this seems alright as there's already a possibility of throwing from this call.

[tmpsantos]

I think it's actually async already despite the API design?

The underlying Map::setStyleJSON so that makes map.load sync too. Note that the exception is thrown at https://github.com/mapbox/mapbox-gl-native/blob/8c2b185ce111cde7c87e588dde8fcdd1eb3e128b/platform/node/src/node_map.cpp#L207

[mikemorris]