When throwing a `MatrixBlockError` the `DUNE_THROW` macro was
abused to also set the data members of `MatrixBlockError` by
referring e.g. to the local (in macro) variable name `th__ex`.
I consider this a bug, since these are undocumented implementation
details. Furthermore their use prohibits any change to `DUNE_THROW`
e.g. to make it `constexpr` friendly.