mark_entry_failed should possibly rollback session

[Elrond]

mark_entry_failed is often called after an exception.
This exception might be an sql exception (it shouldn't, but well, it happens). After an sql exception the session is in a broken state and mark_entry_failed can't do its job and mark the entry failed: If a session is broken, one can't use it for anything, until one cleans it up using .rollback().

So the suggestion is to call Session.rollback() in mark_entry_failed.

Rationale:

1. If the session is clean: fine.
2. If the session is broken: better clean it up instead of having a exception on an exception (harder to read).
3. If someone missed to .save() something, oh well, their fault.

[Elrond]

Doing this is nearly bitesized, IMHO.
I am tagging it.

it could be a big task to write a unit test for this. I don't know, if we want one?
If we want, please remove the bitesized tag!

Chris?