Omnivore fails hard on martian coordinate systems


```
> digest ~/tmp/mars.tif
ERROR 6: Mercator_1SP with scale != 1.0 and latitude of origin != 0, not supported by PROJ.4.
ERROR 6: Mercator_1SP with scale != 1.0 and latitude of origin != 0, not supported by PROJ.4.

/Users/r/gh/mapbox/mapnik-omnivore/lib/raster.js:89
var current = new mapnik.Projection(this.projection);
^
TypeError: please provide a proj4 intialization string
at Raster.getExtent (/Users/r/gh/mapbox/mapnik-omnivore/lib/raster.js:89:17)
at Raster.getCenter (/Users/r/gh/mapbox/mapnik-omnivore/lib/raster.js:78:8)
at /Users/r/gh/mapbox/mapnik-omnivore/index.js:87:12
at pop (/Users/r/gh/mapbox/mapnik-omnivore/node_modules/queue-async/queue.js:24:14)
at /Users/r/gh/mapbox/mapnik-omnivore/node_modules/queue-async/queue.js:38:39
at /Users/r/gh/mapbox/mapnik-omnivore/index.js:82:7
at Object.oncomplete (fs.js:108:15)
```

There may be a general case here: catch errors related to data in coordinate systems that are not recognized by proj4.

An example file in a martian coordinate system is here s3://mapbox/tmp/rclark/mars.tif.

cc @GretaCB

Start with lib/raster.js around line 89 and reproduce the failure using `digest ~/tmp/mars.tif` or the example at s3://mapbox/tmp/rclark/mars.tif. Trace how the error reaches Raster.getCenter and index.js:87. Done means coordinate-system data that PROJ.4 does not recognize is handled without the shown uncaught TypeError.

Written by the indexing model from the issue text.