Well you have output here so you should be able to capture that output and then manually force an error from it.
DECLARE @Output TABLE (OutputMessage NVARCHAR(4000));
SET @sqlop = 'BCP "SELECT blah FROM MYTABLE" queryout "' + @txt4print*
+ '" -b 50000 -c -t"," -S"server" -U"uname" -P"pswd"'
INSERT INTO @Output
exec master..xp_cmdshell @sqlop --, NO_OUTPUT
DELETE FROM @Output WHERE OutputMessage IS NULL
DECLARE @Statement NVARCHAR(MAX)
WHILE (SELECT COUNT(*) FROM @Output) > 0
BEGIN
SELECT TOP 1 @Statement = OutputMessage FROM @Output
IF @Statement LIKE '%Error%'
BEGIN
SET @Statement = 'Unexpected error in procedure: ' + @Statement
RAISERROR(@Statement, 16, 1)
END
END
Alternatively if you just want a message and not an error, you should be able to use PRINT for similar effect.