Handle concurrent updates errors

The C-Service did not handle concurrent update error from PouchDB.

(node:51631) UnhandledPromiseRejectionWarning: Error: Document update conflict.
    at Request._callback (/Users/yannickli/Documents/Concordant/Software/c-service/node_modules/nano/lib/nano.js:168:15)
    at Request.self.callback (/Users/yannickli/Documents/Concordant/Software/c-service/node_modules/request/request.js:185:22)
    at Request.emit (events.js:315:20)
    at Request.EventEmitter.emit (domain.js:467:12)
    at Request.<anonymous> (/Users/yannickli/Documents/Concordant/Software/c-service/node_modules/request/request.js:1154:10)
    at Request.emit (events.js:315:20)
    at Request.EventEmitter.emit (domain.js:467:12)
    at IncomingMessage.<anonymous> (/Users/yannickli/Documents/Concordant/Software/c-service/node_modules/request/request.js:1076:12)
    at Object.onceWrapper (events.js:421:28)
(Use `node --trace-warnings ...` to show where the warning was created)
(node:51631) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:51631) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.