Better handling IO parsing

Very ugly exception are thrown when Funz can't extract values during IO. 
Try to improve message like "Can't get xxx".

IO example: 
```
output.delta_keff.if=true
output.delta_keff.get=grep("jdd.snn.resume", "ECART AU KEFF CIBLE") >> between(") ", "|") >> asNumeric()
output.delta_keff.default=-1
```