Avoid returning without written ctx when posting PR (#31843)

Avoid returning without written ctx when posting PR (#31843)

Fix #31625.

If `pull_service.NewPullRequest` return an error which misses each `if`
check, `CompareAndPullRequestPost` will return immediately, since it
doesn't write the HTTP response, a 200 response with empty body will be
sent to clients.

```go
	if err := pull_service.NewPullRequest(ctx, repo, pullIssue, labelIDs, attachments, pullRequest, assigneeIDs); err != nil {
		if repo_model.IsErrUserDoesNotHaveAccessToRepo(err) {
			ctx.Error(http.StatusBadRequest, "UserDoesNotHaveAccessToRepo", err.Error())
		} else if git.IsErrPushRejected(err) {
			// ...
			ctx.JSONError(flashError)
		} else if errors.Is(err, user_model.ErrBlockedUser) {
			// ...
			ctx.JSONError(flashError)
		} else if errors.Is(err, issues_model.ErrMustCollaborator) {
			// ...
			ctx.JSONError(flashError)
		}
		return
	}
```

Not sure what kind of error can cause it to happen, so this PR just
expose it. And we can fix it when users report that creating PRs failed
with error responses.

It's all my guess since I cannot reproduce the problem, but even if it's
not related, the code here needs to be improved.