The WithEventSource mixin was responsible for catching EventSource
errors and cleaning up events sources then the user left a Controller.
As we are trying to avoid mixin usage, we moved this all to an
`EventSource` component, which can clean up when the component is
removed from the page, and also fires an onerror event.
Moving to a component firing an onerror event means we can also remove
all of our custom computed property work that we were using previously
to catch errors (thrown when a service etc. is removed)
John Cowen