mrconvert: don't set PE scheme if PhaseEncoding::set_scheme throws an exception
mrconvert: don't set PE scheme if PhaseEncoding::set_scheme throws an exception

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