overReactReduxDevToolsMiddleware needs more informative warning when `toJson` not implemented

• Issue Type: FEATURE REQUEST
• over_react Version(s): 3.1.4

When using overReactReduxDevToolsMiddleware with DevToolsStore, if some part of the state, or one of the actions, is not encodable by toJson, the following generic warning is issued:

dart_sdk.js:101547 OverReactReduxDevToolsMiddleware [WARNING]: You must implement a toJson method in your state and actions in order to view state changes in the redux dev tools.

This can be very difficult to track down in a large state tree.

It would be better for the error message to point to the specific object that was not encodable. I have spent a lot of time trying to track it down in various parts of my state tree and actions, when I thought I had implemented toJson properly.

In fact, immediately before I call the constructor for DevToolsStore, I create my state object and call toJson() on it, and it works perfectly, yet I get that warning while the constructor is executing in the next step.

FYI: @greglittlefield-wf @aaronlademann-wf @kealjones-wk @evanweible-wf @maxwellpeterson-wf

Start at overReactReduxDevToolsMiddleware and the DevToolsStore constructor path, then reproduce the warning with a state or action whose toJson is not implemented. Trace how the middleware encodes state and actions; done means the warning identifies the specific non-encodable object rather than only giving the generic message.