It is fairly common practice for C++ programs to have a catch-all for `std::exception` since all STD exceptions and exceptions from many other packages inherit from this class. The VTK-m error classes should also inherit from `std::exception` so that programs can generally manage VTK-m errors without directly interfacing with it. This can be implemented by making `vtkm::cont::Error` inherit from `std::exception`. It also should implement the `what` method to return a const char * for the error message.