When you are using MulTEx, the main paradigm for error handling and reporting are Java exceptions. Any misfunction will be passed to the caller of a method by the standard Java means, an exception. This will be propagated to the upmost level, where it still can be caught, in order to report it. Handling of an individual exception should rarely be necessary.
Depending on the type of user interface, there are different places, where you can catch all propagated exceptions. Sometimes it is not simple to find this place in the UI documentation. Sometimes you will not be able to find it, and must use a workaround in order to assure central exception reporting.
