Make error classes compatible with std::exception

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.