WiX: Preprocessor error and warning

This is simple but could be handy. You can now add preprocessor instructions to you WiX authoring that will allow you to display a warning or error message when the preprocessor encounters that line.

Often errors and warnings would be used inside other prepropcessor constructs.

Example:

<?warning Must define the variable variableThatMustBeDefined ?>
<?ifndef variableThatMustBeDefined ?>
    <?error You did not define the variable variableThatMustBeDefined ?>
<?endif ?>