[feature] Stringify XPCOM errors for messaging system.

[kixxauth]

When an XPCOM error is JSON.stringify()'d to pass through our messaging system it loses all of it's properties. Thus, in the error dialog on the GUI has no useful information for these errors.

So, somehow, we need to detect these errors and stringify them so that they are passed through to the GUI context properly via the messaging system.

However, since the messaging system is still in flux, this will have to wait.