trait edit

Related to <a href="https://ebi-forecast.igb.illinois.edu/redmine/issues/2351">#2351</a>, <a href="https://ebi-forecast.igb.illinois.edu/redmine/issues/2198">#2198</a> and <a href="https://ebi-forecast.igb.illinois.edu/redmine/issues/2245">#2245</a>(only the second part; the first part has already been merged in a previous pull request)
I would still keep this rescue block to handle database constraint violations that we might have neglected to interdict in our Rails code.  Although at this point there are barely any database-level constraints, we are in the process of adding them, and if Rails doesn't catch some violation before it is submitted to the database management system, it would be nice to see an informative error message rather than just going to the generic Rails error page.