Earlier commit a9aa8bf9 had the message but only the test tweak made
it through pre-commit stashing. This is the implementation:
- lib/towerops/lidar/sources/usgs_ned.ex
- lib/towerops/lidar.ex (mosaic_from_ned + get_elevation_from_ned
fallback paths)
- lib/towerops/coverages/building.ex + migration scaffolding
- lib/towerops/workers/coverage_worker.ex error message
- test_helper.exs and the lidar test changes