Construction of a FieldElement from a BigInteger fails if the BigInteger is too big

Currently the method `BigIntegerModulus.reduceModThis()` fails if the BigInteger it gets as input is more than the square of the modulus... However, no exception is thrown if this is the case.

The code should be changed to either throw and exception so the user knows that something went wrong, or fixed to handle the case of large input gracefully.

See this issue https://github.com/aicis/fresco-outsourcing/issues/6