add covariate on trait/edit

see <a href="https://ebi-forecast.igb.illinois.edu/redmine/issues/2198"> redmine #2198</a>

    We only get here if there was an error from trying to update
    attributes of the trait other than the covariate list.  But if
    this happens, we won't see any information about problems with the
    covariates (if there were any).