[material][3pimmiscible] Clang does not compile with DUNE_THROW in constexpr function

!1726 (merged) introduced a DUNE_THROW as default case for a switch in, e.g.,

static constexpr bool isGas(int phaseIdx)

because the compiler would rise a warning otherwise. However, clang does not allow Dune exceptions in constexpr functions. How should we handle this? Just return false per default, omitting the DUNE_THROW?