Improved error reporting for multiple vectors in a model-free analysis.
This was reported by Vitaly Vostrikov (vvostri att uark.edu) at
http://www.mail-archive.com/relax-users@.../msg00991.html (Message-ID:
<CAO2iSXE1ycB4KX1vxqk15CxbW1rpWczZAyR_bX1g6j+yA2PCTQ@...>) and reported at bug #18807
(https://gna.org/bugs/?18807).
