Provide a message to reload the devtools after the extension context is invalidated on an update

[jerelmiller]

When the extension is updated, either automatically by the browser in the background or by the user updating it, the extension context is invalidated so we are no longer able to receive updates from the background script in the devtools script. Currently this results in the UI seeming to "freeze" since there are no updates that we can show in the UI.

#1459 adds support for showing an error screen which will at least now show that something is wrong, but this screen is a bit naive because it captures any error that occurs when communicating with the content scripts.

When an extension is reloaded, there is a specific error thrown when calling port.postMessage that we should be able to detect. When this occurs, let's show a message to the user that the devtools needs to be closed and opened again.