Fix error message in MGPoissonSolver

The following discussion from !327 (merged) should be addressed:

• @snuverink_j started a discussion:

  This message seems completely wrong. Can you fix it (or resolve by opening a new issue?)

            throw OpalException("ArbitraryDomain::compute",
                                "The class ArbitraryDomain only works with parallelization\n"
                                "in z-direction.\n"
                                "Please set PARFFTX=FALSE, PARFFTY=FALSE, PARFFTT=TRUE in \n"
                                "the definition of the field solver in the input file.\n");