Show nested path for style warnings

[aparlato]

Closes #18

This PR updates warnings to show the relative style path to the style with a problem in it. Previously, it warned only on the most nested directory name.

I considered just using the actual style name property, but I think using the path gives users more insight into where to make the change, so I kept that similar to how it was before. Let me know if you disagree here!

QA

• [x] Remove a context from a nested style in the examples (eg https://github.com/stamen/mapbox-gl-style-build/blob/main/examples/nested/templates/styles/dir-1/dir-2/nested-2-A.js#L8)
• [x] Run bin/mapbox-gl-style-build --style-dir=examples/nested/templates/styles --layer-dir=examples/nested/templates/layers --out-dir=examples/nested/build
• [x] Expect a warning to show the nested styles full relative path