show error / success messages for streetmix-loader

[kfarr]

Current behavior:

• No error message provided when streetmix URL is incorrect
• No success message provided when streetmix URL loaded correctly

What is an example of an error case?

• Does the server return a non-200 HTTP status code? (404 or 500)
• Or, did the JSON parser encounter an error?

How to force an error?

• If the URL is a known bad URL (not an actual user or street ID) such as https://streetmix.net/invalid_user/9999

Examples of error messages:

• "Streetmix Loader was unable to load the URL provided: The Streetmix server responded with x HTTP status code."
• "Streetmix Loader was unable to load the URL provided: The JSON parser encountered an error."