Improve user experience around HTTP requests performed by Explorer

[mglaman]

When working with the explorer, the end-user is not aware of HTTP operations happening in the background.

• I wasn't aware that I had to hit "Enter" to fetch the resource when changing the URL.
• When I was on the plane WiFi and received a timeout, I had no idea the Explorer broke
• When I passed an invalid URL which returned a 404, there was no messaging to alert me
• Parsing of the JSON API doc occurs even if it isn't a valid doc (404/403/502) and is a text response.