add covariate on trait/edit

see <a href="https://ebi-forecast.igb.illinois.edu/redmine/issues/2198"> redmine #2198</a>
The problem with this code is that it assumes that the only reason the `save` method might fail is that the covariate value is out of range.  It would be better to use the `save!` method, which throws an exception when it fails rather than returning false.  Then you can set up an exception handler to capture the exception message and store it in the errors variable.  Look for the ActiveRecord::RecordInvalid exception to handle exceptions arising from the validations run by the model.  Look for ActiveRecord::StatementInvalid  to catch exceptions coming from the database management system.  Then you probably want a third block to catch any other exceptions you might not be expecting.