better error message when failing to load gist

[jkulton]

Right now when the gist fails to load from GitHub's API we just show the generalized "Unsupported gist" error.

Instead, we could check the status code of the response.

• If the response was a 200 and the gist has a markdown file, render the gist
• If the response was a 200 and the gist doesn't have a markdown file, render the unsupported gist error
• If the response is a non-200 maybe just render the message property from the API error directly (our users are likely developers, so maybe they would find the error message insightful)