feat(coarse_reverse): add try/catch block around synthesizeDoc code

[missinglink]

fix for the pagerduty alert reported this morning:

2017-09-25_08:03:40.45896 /srv/www/api/releases/20170919173917/node_modules/pelias-model/util/valid.js:29
2017-09-25_08:03:40.45896     throw new PeliasModelError( 'invalid document type, expecting: truthy, got: ' + val );
2017-09-25_08:03:40.45896     ^
2017-09-25_08:03:40.45934 PeliasModelError: invalid document type, expecting: truthy, got: 
2017-09-25_08:03:40.45934     at Object.module.exports.truthy (/srv/www/api/releases/20170919173917/node_modules/pelias-model/util/valid.js:29:11)
2017-09-25_08:03:40.45934     at Document.<anonymous> (/srv/www/api/releases/20170919173917/node_modules/pelias-model/Document.js:255:14)
2017-09-25_08:03:40.45935     at Document.addParent (/srv/www/api/releases/20170919173917/node_modules/pelias-model/Document.js:291:5)
2017-09-25_08:03:40.45935     at /srv/www/api/releases/20170919173917/controller/coarse_reverse.js:70:9
2017-09-25_08:03:40.45935     at Array.forEach (native)
2017-09-25_08:03:40.45935     at synthesizeDoc (/srv/www/api/releases/20170919173917/controller/coarse_reverse.js:69:19)
2017-09-25_08:03:40.45936     at /srv/www/api/releases/20170919173917/controller/coarse_reverse.js:144:23
2017-09-25_08:03:40.45936     at /srv/www/api/releases/20170919173917/node_modules/pelias-microservice-wrapper/service.js:110:18
2017-09-25_08:03:40.45936     at Request.callback (/srv/www/api/releases/20170919173917/node_modules/superagent/lib/node/index.js:679:14)
2017-09-25_08:03:40.45936     at /srv/www/api/releases/20170919173917/node_modules/superagent/lib/node/index.js:868:18
2017-09-25_08:03:41.04725 loading libpostal data, this may take a few seconds...

[orangejulius]

Looks good. nice logging