Exit during forward projection of shapefile extent

[mapsam]

If a shapefile has some sort of invalid projection, it isn't able to forward project the extent value.

mapnik-omnivore/lib/shape.js

Line 99 in 29ead36

extent = transform.forward(extent);

and the process bails out. Let's try/catch this to ensure the file is reprojectable.

cc @GretaCB @millzpaugh

[GretaCB]

@mapsam Good catch. Also, what do you mean by "forward project"?

[mapsam]

From @flippmoke - here's what forward/backward mean in mapnik terms

Projection A;
Projection B;

Transformer T(A, B);

T.forward(data); // A to B of data

T.backward(data); // B to A of data