try to help debug null/undef'd Redux action errors

[cwillisf]

Proposed Changes

This change introduces a new Redux middleware which checks the action and, if it's null or undefined, throws an Error suggesting that this might be due to an invalid state transition.

Reason for Changes

Our current convention for Redux state transitions in GUI involves calls of the form dispatch(createTransitionAction(state)). The createTransitionAction function is expected to check the state to determine whether the transition is allowed, and if not, the function returns undefined.

Unfortunately, the high-level expression of this is difficult for a newcomer to interpret. The output looks similar to this:

Uncaught (in promise) TypeError: Cannot read property 'type' of undefined
    at Object.eval [as dispatch] (webpack-internal:///./node_modules/scratch-gui/node_modules/redux-throttle/build/index.js:56)
    at dispatch (<anonymous>:1:28545)

This error happens inside of redux-throttle because of the way it inspects the action to do its job. With different middleware this error might be expressed differently. Redux itself would probably detect the error and provide more meaningful debugging information, but that would only happen if the empty action made it through all middleware.

After this change, the output looks more like this:

Uncaught (in promise) Error: Missing action in middleware. Invalid state transition?
    at Object.eval [as dispatch] (webpack-internal:///./node_modules/scratch-gui/src/reducers/gui.js:78)
    at dispatch (<anonymous>:1:28545)

In both cases, the stack trace for the exception includes the offending dispatch call, which should help find the failing transition function.

Test Coverage

Tested locally by causing invalid state transitions. Without those, GUI works as expected.