Fix constructors of exceptions
Fix constructors of exceptions

- The const char* constructor is not needed since the variadic template
  constructor covers it already.
- All derived classes now have an explicit 'std::string' constructor to
  call the non-default base class constructor.
- Some code movements
- 'Outlining' of the get_trace() member

MauroCalderara/LinAlg,MauroCalderara/LinAlg,MauroCalderara/LinAlg