Add more checks to postprocessing pipelines

[andreyzhd]

Currently postprocessing pipelines sometimes fail, for all or some of the videos, without notifying the user. Add the checks that the expected number of videos has been created.

[jjnurminen]

I guess you are referring to the Nexus bug where it will no longer produce the videos until it's been restarted? Yea, it would be good to detect this condition. On the other hand, the "postprocessing pipelines" mechanism is intended to be more general, i.e. it should be able to run any operations that require processed data. Checking that videos have been created would only apply to our specific use case of making overlay videos. One possible solution would be to create a Nexus pipeline that checks whether videos have been properly created, and add that to our postprocessing pipelines?

[jjnurminen]

The Nexus pipeline for checking the videos could be written in Python. Alternatively, maybe gaitutils should directly allow Python scripts in preprocessing/postprocessing operations. Perhaps you could e.g. have a rule that if the name of an operation starts with "PY:", it will be interpreted as a Python script to run, instead of a Nexus pipeline.