TSQL try catch transaction error handling, transac

I have a sproc that I am calling from C# with a transaction by doing:

using (var dbContext = PowerToolsDatabase.GetDataContext())
{
    dbContext.Connection.Open();
    using (dbContext.Transaction = dbContext.Connection.BeginTransaction(System.Data.IsolationLevel.Serializable))
    { 
        foreach (var element in package.AddOrUpdateElements)
        {
            dbContext.usp_Element_Commit( /* args */);
        }

        dbContext.Transaction.Commit();
    }
}

And in that sproc there is a try catch, and a RAISERROR inside the try part that is executed under certain circumstances

BEGIN TRY
    BEGIN TRANSACTION   
    SET TRANSACTION ISOLATION LEVEL SERIALIZABLE

    -- Perform Name Uniqueness check (for new)
    IF EXISTS ( SELECT PK.identifier --... )
    BEGIN

        RAISERROR(60000, 16, 1, 'dbo.usp_Element_Commit', 'Supplied Element Name (for new Element) already exists')

        RETURN

    END

    -- Do stuff         

    COMMIT TRANSACTION
END TRY
BEGIN CATCH

    IF XACT_STATE() <> 0
    BEGIN
        ROLLBACK TRANSACTION;
    END

    DECLARE @ErrorMessage NVARCHAR(4000);
    DECLARE @ErrorSeverity INT;
    DECLARE @ErrorState INT;

    SELECT
        @ErrorMessage = 'dbo.usp_Element_Commit -- ' + ERROR_MESSAGE(),
        @ErrorSeverity = ERROR_SEVERITY(),
        @ErrorState = ERROR_STATE();

    RAISERROR (@ErrorMessage, @ErrorSeverity, @ErrorState);

END CATCH;

When I run that and hit the RAISERROR inside the try section of the sproc I get the following multiple errors:

dbo.usp_Element_Commit -- Supplied Element Name (for new Element) already exists
Transaction count after EXECUTE indicates a mismatching number of BEGIN and COMMIT statements. Previous count = 1, current count = 0.
Transaction count after EXECUTE indicates a mismatching number of BEGIN and COMMIT statements. Previous count = 1, current count = 0.

What is the recommended way to handle errors and and jump to the catch block without messing up the external transaction?

Also if I remove the rollback from the catch block in the sproc then I get the same Transaction Count message with Previous Count = 1, current count = 2

I changed my sproc to follow this pattern of using either a save point or a transaction depending on if there is already an existing transaction. And it works as expected now.

BEGIN TRY
    declare @trancount int = @@trancount
    if @trancount = 0
        begin transaction
    else
        save transaction usp_element_commit_transaction;

    set transaction isolation level serializable

    -- Perform Name Uniqueness check (for new)
    IF EXISTS ( SELECT PK.identifier --... )
    BEGIN

        RAISERROR(60000, 16, 1, 'dbo.usp_Element_Commit', 'Supplied Element Name (for new Element) already exists')

        RETURN

    END

    -- Do stuff         

    COMMIT TRANSACTION
END TRY
BEGIN CATCH

    declare @xstate int = xact_state()
    if @xstate = -1
        rollback
    else if @xstate = 1 and @trancount = 0
        rollback
    else if @xstate = 1 and @trancount > 0
        rollback transaction usp_element_commit_transaction;

    DECLARE @ErrorMessage NVARCHAR(4000);
    DECLARE @ErrorSeverity INT;
    DECLARE @ErrorState INT;

    SELECT
        @ErrorMessage = 'dbo.usp_Element_Commit -- ' + ERROR_MESSAGE(),
        @ErrorSeverity = ERROR_SEVERITY(),
        @ErrorState = ERROR_STATE();

    RAISERROR (@ErrorMessage, @ErrorSeverity, @ErrorState);

END CATCH;