fix(action): fix unhandled promise rejection

[benatshippabo]

Summary

This fixes the unhandled promise rejection when something errors out inside main. An example would be when one of the git commands fail such as pushing to a write protected branch, you would see this message but the action still passes:

(node:1625) 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(). (rejection id: 1)
(node:1625) [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.

It is fixed by using core.setFailed documented here.

Related issues: #2 #9

[Wattenberger]

this is great, thanks for the PR!