If you print something on the error output and the process returns with an error then the error message is displayed in the CLI module GUI:
Progress reporting is described here: Documentation/Nightly/Developers/SlicerExecutionModel - Slicer Wiki
If you print something on the error output and the process returns with an error then the error message is displayed in the CLI module GUI:
Progress reporting is described here: Documentation/Nightly/Developers/SlicerExecutionModel - Slicer Wiki