Deal with PluginErrors that are not raised

[tlambert03]

In #35, @Carreau introduced some nice code to make sure that plugin errors are raised, so that their tracebacks look better. I merged it too hastily. It's a good goal, but we actually use non-raised PluginErrors in too many places. That needs to change as well before we can incorporate it. I reverted it, but we shouldn't forget it.