Exit with error code when encountering issues

[felixLam]

Right now elm-analyse always exits with status 0.

This makes it more difficult to run it in a CI setting where we want to mark a build as unstable for certain conditions.

It would be nice if the exit code was only 0 for runs without encountering any issues.

At a minimum it should be non-zero when encountering issues. Nice to have would be a configurable threshold for "unstable"/"failed". We could then exit with status code 1 if any issues were encountered and status code 2 if the threshold it exceeded.