Improve error message when updating labels without `pull-requests:write` permissions

[Kevin-de-Jong]

Scope

The following error is raised when running this action with update-labels: true (default behavior), without having pull-requests:write:

Resource not accessible by integration

Proposal

Provide a nice error message to the end user, explaining which permission needs to be granted to your token;

❌  CommitMe is configured to update Pull Request labels but does not have the right permissions to do so (`pull-requests:write`).

[Achllle]

I've found that both issues: write and pull-requests: write were needed for the action to run. See this PR. The last few commits show that I tried with just pull-requests: write, which failed with an API issue from Github, then saw this comment and tried with just pull-requests: issues, which failed with a different error. Then it worked when I added both. The official docs say that either of those should work.