Improve error message if no source folder for generating images is selected

[matthias-ronge]

If a task is ended, which has the "generate images" option, but no folder is defined as the source in the project, the task crashes with an unclear error message in the task manager: Crashed (searching for files)

Stacktrace:

java.lang.NullPointerException: null
    at org.kitodo.production.model.Subfolder.determineDirectoryAndFileNamePattern(Subfolder.java:172)
    at org.kitodo.production.model.Subfolder.listContents(Subfolder.java:311)
    at org.kitodo.production.model.Subfolder.listContents(Subfolder.java:295)
    at org.kitodo.production.services.image.ImageGenerator.determineSources(ImageGenerator.java:216)
    at org.kitodo.production.enums.ImageGeneratorStep$1.accept(ImageGeneratorStep.java:38)
    at org.kitodo.production.enums.ImageGeneratorStep$1.accept(ImageGeneratorStep.java:33)
    at org.kitodo.production.services.image.ImageGenerator.run(ImageGenerator.java:348)
    at org.kitodo.production.services.data.TaskService.generateImages(TaskService.java:536)
    at org.kitodo.production.thread.TaskScriptThread.run(TaskScriptThread.java:69)

Goal: Possible suggestions for improvement:

• Task cannot be completed with incomplete project configuration (error message when trying to complete)
• Clearly formulate the error message in the Task Manager