shp2json should ignore EPIPE errors on write.

[mbostock]

For example:

$ shp2json -n example.shp | head -n2
{"type":"Feature","properties":{"FID":0},"geometry":{"type":"Point","coordinates":[1,2]}}
{"type":"Feature","properties":{"FID":1},"geometry":{"type":"Point","coordinates":[3,4]}}
events.js:160
      throw er; // Unhandled 'error' event
      ^

Error: write EPIPE
    at exports._errnoException (util.js:1012:11)
    at WriteWrap.afterWrite (net.js:793:14)

If stdout throws an EPIPE exception we should just swallow it silently and abort.