Add user-visible error handling when map fails to load

[eugenethreat]

Related to #9 - if loading data fails, the map just stays blank. Adding user-visible error handling (a popup saying 'loading map data failed' could be a nice touch.

[hawc2]

Is this still an issue? Can't recall last time I saw it happen

[eugenethreat]

I haven't seen it happen either but it's good practice to have error handling in case it does happen again - for example if the map data API hosted on Render goes down for some reason. It would be a good first issue for someone to tackle as well. I am in favor of leaving it open.

[hawc2]

I think it's happening in this current pull request #85. An error message would be helpful for this sort of debugging

[eugenethreat]

I think it's happening in this current pull request #85. An error message would be helpful for this sort of debugging

Error message was happening due to syntax error w/ settting MAPBOX_API, so not necessarily an error with loading the map content, but accessing it in the first place. We could still add some sort of error handling for all scenarios?