Handle Mapnik errors when creating VT

[GretaCB]

With recent Vector Tile v2 spec updates, Mapnik is now throwing on errors that previously did not throw.

This PR helps handle these errors gracefully with a more explicit error code EINVALID for expected errors (errors that do not need to throw).

cc @springmeyer @BergWerkGIS @rclark

[coveralls]

Coverage increased (+0.09%) to 94.163% when pulling 6ccb6e4 on handle-error into 15e4501 on master.