Add notification function

[s-aebischer]

Is your feature request related to a problem? Please describe

When a job fails or gets canceled the main application can't handle this change. It would be nice to simply add a handler function in which the main application can handle the error (e.g. add an other job).

Describe the solution you'd like

When a job status is changed a function should be called in the main application. In this method custom error handling can be added.