Style cleanup

[remi-dupre]

Attempt to cleanup a bit existing python scripts, though my humble point of perception.

What changed

• Applied black formater and integration in the CI, like in Idunn
• Tried to fix large string defined in tasks.py breaking the identation
• Propagate the exceptions raised from concurrent tasks
• Simplify exec.py (it lost ~100 lines of code, and may be more maintainable and understandable from my point of view?)