Exit during forward projection of shapefile extent

If a shapefile has some sort of invalid projection, it isn't able to forward project the extent value. https://github.com/mapbox/mapnik-omnivore/blob/29ead3673df622c773f334ec555d3d749a9c673f/lib/shape.js#L99 and the process bails out. Let's try/catch this to ensure the file is reprojectable.

cc @GretaCB @millzpaugh

Inspect lib/shape.js around line 99, where forward projection of the shapefile extent can terminate processing for an invalid projection. Add handling so the file remains reprojectable instead of bailing out, then verify that an invalid projection no longer exits the process.

Written by the indexing model from the issue text.