Simplify error code handling for external pipeline validation

Prior to this, the external pipeline validator would reject a pipeline if the service returned any 4xx error code, but the behavior was different for GitLab.com, where only a 406 error code would be used to reject.

We simplify the handling of this error code by making 406 the only error code to signify a rejection. This not only makes it easier to understand, but it also makes the behavior consistent from one GitLab instance to another.

We had to work around this issue in the validator by returning a 500 error when a 4xx was warranted.