Show a "didn't load" broken icon next to a layer if it fails to fetch data

[crisner]

Sometimes layers fail to fetch data from its API. In such cases show a broken icon next to the layer in the layer menu to visually indicate that the data fetch was not successful.

We could separately decide to show a "didn't load" broken icon like https://fontawesome.com/v4.7.0/icon/exclamation-triangle next to the layer name!

Quoting @jywarren from #290 (comment)

[neelesh17]

@crisner is this issue still avalible for claiming.

[crisner]

I am working on this today @neelesh17. Would you like to help add a new layer like this issue #122 or help write tests for layers? If you would like to help write tests I can create an issue for you and you could follow the tests that have already been written. Let me know what you are interested to help with. Thanks!

[neelesh17]

I would like to help you with issue #122.

[crisner]

Excellent! Shall I mention on the issue that you've claimed it?

[neelesh17]

Yeah, that would be great.