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

Fix go-gitea#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.

routers/web/repo/pull.go

@@ -1296,9 +1296,10 @@ func CompareAndPullRequestPost(ctx *context.Context) {
 	// instead of 500.
 
 	if err := pull_service.NewPullRequest(ctx, repo, pullIssue, labelIDs, attachments, pullRequest, assigneeIDs); err != nil {
-		if repo_model.IsErrUserDoesNotHaveAccessToRepo(err) {
+		switch {
+		case repo_model.IsErrUserDoesNotHaveAccessToRepo(err):
 			ctx.Error(http.StatusBadRequest, "UserDoesNotHaveAccessToRepo", err.Error())
-		} else if git.IsErrPushRejected(err) {
+		case git.IsErrPushRejected(err):
 			pushrejErr := err.(*git.ErrPushRejected)
 			message := pushrejErr.Message
 			if len(message) == 0 {
@@ -1315,7 +1316,7 @@ func CompareAndPullRequestPost(ctx *context.Context) {
 				return
 			}
 			ctx.JSONError(flashError)
-		} else if errors.Is(err, user_model.ErrBlockedUser) {
+		case errors.Is(err, user_model.ErrBlockedUser):
 			flashError, err := ctx.RenderToHTML(tplAlertDetails, map[string]any{
 				"Message": ctx.Tr("repo.pulls.push_rejected"),
 				"Summary": ctx.Tr("repo.pulls.new.blocked_user"),
@@ -1325,7 +1326,7 @@ func CompareAndPullRequestPost(ctx *context.Context) {
 				return
 			}
 			ctx.JSONError(flashError)
-		} else if errors.Is(err, issues_model.ErrMustCollaborator) {
+		case errors.Is(err, issues_model.ErrMustCollaborator):
 			flashError, err := ctx.RenderToHTML(tplAlertDetails, map[string]any{
 				"Message": ctx.Tr("repo.pulls.push_rejected"),
 				"Summary": ctx.Tr("repo.pulls.new.must_collaborator"),
@@ -1335,6 +1336,11 @@ func CompareAndPullRequestPost(ctx *context.Context) {
 				return
 			}
 			ctx.JSONError(flashError)
+		default:
+			// It's an unexpected error.
+			// If it happens, we should add another case to handle it.
+			log.Error("Unexpected error of NewPullRequest: %T %s", err, err)
+			ctx.ServerError("CompareAndPullRequest", err)
 		}
 		return
 	}