build: improve exception handling
build: improve exception handling

- don't try to handle unexpected exceptions, let default handler operate
  so we get full backtrace, etc.

- don't raise Exception instances directly, since these can't be caught
  without also potentially catching all manner of other exceptions. Want
  handling to be specific to the error we raised ourselves.

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