Fixes missing return value in non-void function of WellsManager.

gcc warned about the following

/home/mblatt/src/dune/opm/opm-core/opm/core/wells/WellsManager.cpp: In function ‘std::array<long unsigned int, 3ul> WellsManagerDetail::directionIndices(Opm::CompletionDirection::DirectionEnum)’:
/home/mblatt/src/dune/opm/opm-core/opm/core/wells/WellsManager.cpp:191: warning: control reaches end of non-void function

To calm it I introduced a default branch in the switch statement that throws.
This should of course never be reached.