Exception: New function push_back (const Exception&)
Exception: New function push_back (const Exception&)

This allows concatenation of exceptions when the concatenation may not be in a simple stack order.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3