Change color to red if the personal access token is invalid

[vfonic]

I just tried setting invalid github token and realized that I didn't get any errors back.

This happens here: https://github.com/fabiospampinato/vscode-github-notifications-bell/blob/master/src/statusbar.ts#L70

When API token is correct, the response is an array of notifications: JSON.parse ( result[0].body ) // => [{...}, {...}]

When API token is incorrect, the response is an object: JSON.parse ( result[0].body ) // => {"message":"Bad credentials","documentation_url":"https://developer.github.com/v3"}

It would be useful for user to know that they didn't set the token right.