[Cleanup] Introduce a tiny bit more debugging data to warnings about CP dependencies

[trek]

I ran across this updating an app from 2.4LTS to 2.5.

We started with code a bit like this:

const foos = 'abc';
computed('model.@each', foos, function() {})

And mutated to:

const foos = ['abc', 'def'];
// !!! -> `foos` not properly tracking because it is now an array, not a string. 
computed('model.@each', foos, function() {})

Then we started the 2.4 -> 2.5 update in our app.

In 2.5, Ember started protecting from this error by checking typeof for dependencies and asserting if you passed other types. It was a little rough to track down with only "A computed property key must be a string" as a guide.

This PR updates the assertion with a tiny bit more debugging data.

[rwjblue]

Awesome, thank you @trek!

[trek]

👋 long time no commit.

[locks]

Such emeritus 😘