Fix api error message if fork exists (#24487) (#24493)

Backport #24487 by @fnetX

On the @Forgejo instance of Codeberg, we discovered that forking a repo
which is already forked now returns a 500 Internal Server Error, which
is unexpected. This is an attempt at fixing this.

The error message in the log:
~~~
2023/05/02 08:36:30 .../api/v1/repo/fork.go:147:CreateFork() [E]
[6450cb8e-113] ForkRepository: repository is already forked by user
[uname: ...., repo path: .../..., fork path: .../...]
~~~

The service that is used for forking returns a custom error message
which is not checked against.

About the order of options:
The case that the fork already exists should be more common, followed by
the case that a repo with the same name already exists for other
reasons. The case that the global repo limit is hit is probably not the
likeliest.

---------

Co-authored-by: Otto Richter (fnetX) <git@fralix.ovh>

routers/api/v1/repo/fork.go

@@ -141,7 +141,7 @@ func CreateFork(ctx *context.APIContext) {
 		Description: repo.Description,
 	})
 	if err != nil {
-		if repo_model.IsErrReachLimitOfRepo(err) || repo_model.IsErrRepoAlreadyExist(err) {
+		if repo_service.IsErrForkAlreadyExist(err) || repo_model.IsErrRepoAlreadyExist(err) || repo_model.IsErrReachLimitOfRepo(err) {
 			ctx.Error(http.StatusConflict, "ForkRepository", err)
 		} else {
 			ctx.Error(http.StatusInternalServerError, "ForkRepository", err)