A small change to clarify the error message produced by bug #8059 (https://gna.org/bugs/?8059).
See the post at https://mail.gna.org/public/relax-devel/2006-12/msg00027.html (Message-id:
<7f080ed10612210231l17934be5k7b614ab38f1b09d@...>) for more details.
Instead of returning None when no diffusion tensor data is present, a RelaxNoTensorError is raised.
