Repository Dispatch: improve error message for fork PRs

[AgnesToulet]

The RepositoryDispatch action is used by the Enterprise PR check to trigger a build on Enterprise to test OSS changes against Enterprise codebase. This is not working for forks as forks don't have access to the GH token to trigger the event and this is confusing for a lot of people that don't know if they should merge or not the PR. Adding this log should help reviewers understand why this error is happening and what is expected from them.

This PR also updates TS dependencies as before 4.6 and this change, it wouldn't allow code to be called before super().

@xlson adding you as a reviewer to check the log message, is it clear enough?
Example of how it appears in GH: https://github.com/grafana/grafana/runs/6709763832?check_suite_focus=true

[xlson]

I think it's clear enough. I wonder if we should be adding a link to more information but I think we can do that later if we need to, better to get this merged now and see what effects it has.

[tolzhabayev]

Just some comments, if tested and it really works then LGTM!