Warn user if required sprite property is not present or non-existent image is requested from sprite

[ryanhamley]

Launch Checklist

• briefly describe the changes in this PR
  • logs a console warning if sprite URL is not defined and a property that requires sprites is used
  • logs a console warning if an image not in the sprite is requested
• write tests for all new functionality
  • i don't see tests for the existing glyph validation. do we think unit tests are necessary for this? if so, i can add them for both sprite and glyph validation
• manually test the debug page
  
  

Closes #6823

[mollymerp]

nice! the style-spec validation tests are a little different from our other unit tests, and use fixtures.
here are the relevant fixtures for the missing glyphs validation:
https://github.com/mapbox/mapbox-gl-js/blob/master/test/unit/style-spec/fixture/missing-glyphs.input.json
https://github.com/mapbox/mapbox-gl-js/blob/master/test/unit/style-spec/fixture/missing-glyphs.output.json

we also want to be careful and make sure this error doesn't fully break existing styles that fail this validation check. from what I can tell right now it does block rendering if the validation fails on the initial style load, but it doesn't break if the validation fails with addLayer or set*Property

[ryanhamley]

Closing in favor of #7987

[ansis]

@ryanhamley I wasn't aware of this pr, sorry. Do you think it makes sense to still add the warning for missing icons to the other pr? (in the case the user doesn't add the missing icon in the event's callback)

[ryanhamley]

No problem. It's something I quickly took a stab at awhile ago and then never went back to. The goal was always a more fleshed out option like this. A warning might be useful because right now you wouldn't necessarily know that anything was wrong if you weren't listening for the event. I can imagine spending a few hours tearing my hair out trying to figure out why my image wasn't showing up only to realize there's a typo or something.