Make messaging plugin loading more pleasant for dev
    
    The way the messaging plugins are loaded, it is required to run
    `python setup.py develop` for resultsdb to function properly,
    if you use it from git checkout.
    
    To make the development a bit more pleasant, I made sure the
    dummy plugin can be loaded even without that.
    
    Test Plan:
    As @ralph noted at IRC, doing this kind of lets us in the dark most
    of the time, so to catch the plugin loading issues, I updated the
    unitests with code that tries to load both dummy (should work always)
    and fedmsg (only works after `setup.py develop`) plugins.
    Helpfull message is shown when loading the fedmsg plugin fails.
    
    Fixes T883
    Differential Revision: https://phab.qa.fedoraproject.org/D1087