Generating Custom SQL Errors

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SQL

Hey guys i need your help, i am trying to generate custom sql error messages, but its not working very wull.
There are 2 custom generated messages ErrorMsg and ErrorMsg1, ErrorMsg workes fine, but ErrorMsg1 doesn't. Somehow it keep getting overwritten by the default sql error message.

And sorry about the sql, the space allotted is not sufficient for proper spacing of the sql.

USE [AssetTracker]
GO
/****** Object:  StoredProcedure [dbo].[Populate_Asset_Table]    Script Date: 11/11/2013 13:17:28 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO

--DROP Procedure [dbo].[Populate_Asset_Table]
ALTER Procedure [dbo].[Populate_Asset_Table]

@AssetIdentifier varchar(50),
@AssetCondition int,
@SchoolID int,
@AssetCategoryID int,
@VendorID int,
@AssingedPersonID int,
@AssetStatusID  int,
@ManufacturerID int,
@ModelDetail varchar(50),
@CreatedOn datetime,
@Notes varchar(max)

AS

DECLARE @CreatedByIdentifier nvarchar(50),@ModifiedByIdentifier nvarchar(50),@ModifiedOn datetime,@ErrorMsg nvarchar(500)

    SET @CreatedByIdentifier  =(SELECT SUSER_NAME())
    SET @ModifiedByIdentifier =(SELECT SUSER_NAME())
    SET @ModifiedOn           = (SELECT GETDATE())
    SET @ErrorMsg = 'The Asset with this identifier:'+@AssetIdentifier +',already exists in this table.'
    
BEGIN

    
	IF EXISTS(SELECT AssetIdentifier FROM AssetTracker_Asset WHERE AssetIdentifier = @AssetIdentifier)
	   BEGIN
		RAISERROR(@ErrorMsg, 10, 1) --change to > 10
		RETURN --exit now
	   END
    ELSE
	   BEGIN	
	      BEGIN TRY
                   INSERT INTO AssetTracker_Asset VALUES(@AssetIdentifier,@AssetCondition,@SchoolID,@AssetCategoryID,@VendorID,@AssingedPersonID,
				@AssetStatusID,@ManufacturerID,@ModelDetail,@CreatedOn,@CreatedByIdentifier,@ModifiedOn,@ModifiedByIdentifier,@Notes)
	      END TRY
	      BEGIN CATCH
		DECLARE @ErrorNum int, @ErrorMessage nvarchar(4000), @ErrorMsg1 nvarchar(500)
		SELECT @ErrorNum = ERROR_NUMBER()
		SELECT @ErrorMessage = ERROR_MESSAGE()
		SET @ErrorMsg1 ='Error Number:'+@ErrorNum + 'And Error Message:'+@ErrorMessage +'. Occured in Populate_Asset_Table while inserting these elments'
			    
	        RAISERROR (@ErrorMsg1,10,1) 
    	        RETURN
	      END   CATCH
             END
END

Thanks for your help