Tangram-play doesn't report an error when tiles cannot load because of bad URL

[skorasaurus]

While loading one of Patricio's scenes, (https://raw.githubusercontent.com/tangrams/tangram-sandbox/48ee0e7901a06818eb5fe15866171f1239a690ce/examples/ink.yaml);
The map didn't display in tangram-play but there was no error.

I did a quick compare with another scene file and noticed it was missing HTTPS in the url. Once I added HTTPS to the url; it loaded fine.

For future users, adding an error when tiles can't load would be helpful.

ref: #533

[louh]

Thanks @skorasaurus. There's a whole category of tile load errors that Tangram.js logs which are not expressed through its usual error event handler, which is how applications like Tangram Play can know about them. This is something we're hoping to improve soon.