issue #39: update error message
issue #39: update error message

CompEvol/beast2,tgvaughan/beast2,tgvaughan/beast2,CompEvol/beast2,Anaphory/beast2,Anaphory/beast2,tgvaughan/beast2,CompEvol/beast2,Anaphory/beast2,Anaphory/beast2,tgvaughan/beast2,CompEvol/beast2