Action silently succeeds if postman collection file does not exist

[nt-gt]

While trying to setup a GitHub CI using newman-action, I noted that the CI succeeded when I passed it a non-existing postman collection without any warnings what so ever. I think newman-action should produce a hard error by default if you pass it invalid parameters (I also passed -1 as iterationCount by mistake as well without any errors - for me, #242 would have avoided this problem for me as well).

The only reason why I discovered this is because I was surprised by the run time being so low.