Merge retry_failed.py into main.py

[amzid]

We often needed to adapt changes made in main.py to retry_failed.py (see #256 e.g.) after some code changes in other parts.
Many people are also not aware of this file or are not sure about how or when to use it.
I suggest to fuse both files into one single file main.py.

[jdsika]

@amzid I think this can be closed.