how to make a function inside a catch block?

Question

Im a beginner in coding so please bare with my lack of knowledge, thank you.

i have a code initially looking like this in my catch block

            catch (Exception ex)
            {
                String[] msg = ex.Message.Split(new char[] { '|' });
                if (msg[0] == "EXIST")
                {
                    lblStatus.CssClass = "Error";
                    lblStatus.Text = "<span class='" + Resources.Resource.LangParam2 + "'>" + Resources.Resource.UserNameAlreadyExists + "</span>";

                    return;
                }
                else
                {
                    throw ex;
                }
            }

i am trying to make the code in the catch function into a library function as many code will use it. for example I am trying smth like this

            catch (Exception ex)
            {
                String[] msg = ex.Message.Split(new char[] { '|' });
                Util.GetErrorMessage("SubUser1");
                else
                {
                    throw ex;
                }
            }

and inside my Util.cs code is as below

        public static void GetErrorMessage(string acc)
        {
            if(acc == "SubUser1")
            {
                String[] msg = new string[] { };
                if (msg[0] == "EXIST")
                {
                    string Class = "Error";
                    string Text = "<span class='" + Resources.Resource.LangParam2 + "'>" + Resources.Resource.UserNameAlreadyExists + "</span>";

                    return;
                }
            }

        }

any idea that i can make this happen? thank you!