Is there a way to have multiple error messages displayed with RAISERROR conditionally?
One method is to build an error message string and issue a single RAISERROR conditionally after all validations are done.

DECLARE @ErrorMessage varchar(2047);
IF @ForeCastYearCheck = 'false'
    SET @ErrorMessage = @ForeCastYearCheckError;
IF @ForeCastQuarterCheck = 'false'
    SET @ErrorMessage = COALESCE(@ErrorMessage + ',' + @ForeCastQuarterCheckError, @ForeCastQuarterCheckError);
IF @ForeCastComboCheck = 'false'
    SET @ErrorMessage = COALESCE(@ErrorMessage + ',' + @ForeCastComboCheckError, @ForeCastComboCheckError);
IF @ErrorMessage IS NOT NULL
    RAISERROR(@ErrorMessage, 16, 1);