more informative error message
more informative error message

vlas-sokolov/pyspeckit,mikelum/pyspeckit,pyspeckit/pyspeckit,mikelum/pyspeckit,keflavich/pyspeckit,e-koch/pyspeckit,keflavich/pyspeckit,allisony/pyspeckit,low-sky/pyspeckit,e-koch/pyspeckit,low-sky/pyspeckit,jpinedaf/pyspeckit,allisony/pyspeckit,jpinedaf/pyspeckit,vlas-sokolov/pyspeckit,pyspeckit/pyspeckit