Handling exceptions in the Events panel

[stumitchell]

Currently traced events can raise exceptions and still have their trace shown in the panel.

However the Exception isn't displayed, and there is no visual indication that something went wrong (apart from half of the trace missing)

If the exception is shown the trace screen would be a fantastic help to determine what went wrong as it would show the values of the fragments that lead up to the error.

I suggest that the form that caused the exception should be traced and displayed in red.
and the value of that form should be the error and it should be displayed somewhere by default.

day8.re-frame/tracing will need to be changed to take this into account.

see day8/re-frame-debux#34