Use "why not code this" bubbles for errors and warnings ("Fix-its")

[uliwitness]

In #30 we mention iPhoto-like bubbles that show suggestions from the IDE (to allow for more exploratory coding without having to read manuals etc.).

These could also be used for certain "warnings" in code. E.g. if you use a command that we know can fail, we could show such a bubble on that line and offer to add the skeleton for error handling there (like if the result = "no such card" then displayMyErrorDialog or whatever).

This would be mainly aimed at encouraging good practices, like catching unused results, unhandled errors, not annotating handlers with the errors they may return etc., but is otherwise very similar to the errors/warnings in Xcode with the "Fix it" buttons next to them.