feat(plugins): for signallling error handling

[pekasen]

Right now, plugins are not expected to raise Exceptions. But in case of a process, e.g. retrieval of API information, fails it is unknown to ponyexpress. Thus, for better handling we should introduce at least two different exception classes, one for a recoverable error (e.g. for signalling that a retry should be scheduled) and one for terminating the task entirely (setting the failed state for a task).