Non-descriptive error msg when trying to parse unknown relations

```
> cat lala.cat 
let a = b & rf
acyclic a

> java -jar dartagnan/target/dartagnan.jar lala.cat benchmarks/locks/ttas.c
Test: benchmarks/locks/ttas.c
Result: ERROR
Reason: IllegalArgumentException
Details:
        Non-unary relation in [[b] := b, [rf] := rf]
```