Exit the process with error code when unhandled rejection

[marvinroger]

Running whcli with, for example, an invalid token, exits with 0, e.g.:

whcli forward --token=faceb236-ad99-463c-bd78-57e872d18b31 --target=http://localhost

This PR adds a process.exit(1); to the unhandledRejection handler, which is the default behavior in Node.js.
Also, I removed the process.abort() which I think is never called because the following process.exit runs right away.