if repo does not exist, show a 404 not a 500 (#5900)

Author: adelowo

routers/user/home.go

@@ -287,7 +287,10 @@ func Issues(ctx *context.Context) {
 	if repoID > 0 {
 		if _, ok := showReposMap[repoID]; !ok {
 			repo, err := models.GetRepositoryByID(repoID)
-			if err != nil {
+			if models.IsErrRepoNotExist(err) {
+				ctx.NotFound("GetRepositoryByID", err)
+				return
+			} else if err != nil {
 				ctx.ServerError("GetRepositoryByID", fmt.Errorf("[%d]%v", repoID, err))
 				return
 			}