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 throw clause after the switch statements. Thus adding a new
enum value will raise a warning on smart compilers, hopefully.
Merge pull request #684 from blattms/fixes-missing-return-value-in-wellsmanager

Fixes missing return value in non-void function of WellsManager.
[cleanup] Removes a superfluous if-check of the well state.

There were to identical if statements and the second one was followed
by an else branch. While in this case (if statement just throws) it is not
a bug, this commit cleans up one of the if statements.
Merge pull request #687 from blattms/remove-superlfluous-if-in-wellstate-check

[cleanup] Removes a superfluous if-check of the well state.