Fix Server Crash on Undeliverable Challenge Solved Webhook

[J12934]

This can happen when the configured domain doesn't exist or doesn't respond at all.

This leads to errors like the following:

info: Solved challenge Score Board (Find the carefully hidden 'Score Board' page.)
error: Webhook notification failed: connect ECONNREFUSED 10.110.238.153:80
/juice-shop/lib/webhook.js:33
      throw error
      ^

Error: connect ECONNREFUSED 10.110.238.153:80
    at TCPConnectWrap.afterConnect [as oncomplete] (net.js:1141:16) {
  errno: 'ECONNREFUSED',
  code: 'ECONNREFUSED',
  syscall: 'connect',
  address: '10.110.238.153',
  port: 80
}
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! juice-shop@12.0.2 start: `node app`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the juice-shop@12.0.2 start script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /home/juicer/.npm/_logs/2020-11-20T17_49_42_309Z-debug.log

Had to do some bigger changes than I expected so that the function properly throws and catches the async error and are usable in the server tests.

[bkimminich]

ℹ️ Unfortunately the Travis-CI pipeline is still unavailable due to missing OSS credits. I've pinged them on the respective support ticket once again, but haven't heard back from them yet. Reviews and merging of open PRs will resume once our CI/CD is back online. Thanks for your contribution and patience! 👍