[git-webkit] Improve the error message when PR fails to create

[Tim Nguyen (:ntim)]

Sometimes the PR fails to create because the branch needs rebasing, but the error message is:

```
Validation failed when creating pull request
Does a pull request against this branch already exist?
```

[Radar WebKit Bug Importer]

<rdar://problem/98218733>

[Tim Nguyen (:ntim)]

actually my PR didn't fail because of rebasing, it was because the commit message was too long:

{'message': 'Validation Failed', 'errors': [{'resource': 'Issue', 'code': 'custom', 'field': 'body', 'message': 'body is too long (maximum is 65536 characters)'}], 'documentation_url': 'https://docs.github.com/rest/reference/pulls#create-a-pull-request'}

[Tim Nguyen (:ntim)]

Pull request: https://github.com/WebKit/WebKit/pull/3068

[Youssef Soliman]

Pull request: https://github.com/WebKit/WebKit/pull/3069

[EWS]

Committed 253428@main (b33f467f3096): <https://commits.webkit.org/253428@main>

Reviewed commits have been landed. Closing PR #3069 and removing active labels.

[EWS]

Committed 252432.225@safari-7614-branch (7bbd78c8b752): <https://commits.webkit.org/252432.225@safari-7614-branch>

Reviewed commits have been landed. Closing PR #32 and removing active labels.