"fix" for issue 122: raise useful exceptions
"fix" for issue 122: raise useful exceptions

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