Not using "ctx.ServerError" in api (#12907)

This function will render a whole html page which is not useful for API.

Signed-off-by: a1012112796 <1012112796@qq.com>

routers/api/v1/repo/fork.go

@@ -109,7 +109,7 @@ func CreateFork(ctx *context.APIContext, form api.CreateForkOption) {
 		}
 		isMember, err := org.IsOrgMember(ctx.User.ID)
 		if err != nil {
-			ctx.ServerError("IsOrgMember", err)
+			ctx.Error(http.StatusInternalServerError, "IsOrgMember", err)
 			return
 		} else if !isMember {
 			ctx.Error(http.StatusForbidden, "isMemberNot", fmt.Sprintf("User is no Member of Organisation '%s'", org.Name))