improved error handling and reporting

I'd like to see more informative error messages, including %errmsg set on every error, the values for %err defined in each action's documentation, and something more than Err displayed in the RunLog.  I think the message displayed in the editor when a plugin has an error could also be made more useful, as could the warnings shown on startup.

I'd also like to see more error handling, so the steps of saving, examining, and clearing each error don't have to be recreated for each potentially failing action.