Improved plot errors

[mrerrormessage]

• Pulls problems with widgets up into compilation.{success,messages}, for easier access in Galapagos. This will require Galapagos checking code to use these new keys.
• Uses modelConfig.output.alert("alert message here") to notify of errors within plots and pens. Fails gracefully where possible (eg. a bad pen will not wreck the plot that contains it). This change will require providing modelConfig.output.alert in Galapagos.
• Empty plot and pen methods are compiled to function() {} instead of the longer function with context nesting. This is done as a single commit in case it is not desired, and required 3 lines of change here. The reason I would favor making this change is that it makes some source dumps, this one for instance, much easier to audit for changes.

[mrerrormessage]

Doh! Forgot to associate with existing issue #160. I can re-open this as a PR for that issue, or we can close this one. Either way, they're the same thing.