Add an "Invalid Response Format" error type for login (and all other endpoints)

Many poorly-built plugins return HTML from the JSON endpoints, which we obviously can't parse. There's no good way for us to deal with this, but we can have an application-layer error that indicates that the server is returning nonsense. It should contain the raw response so that the host app can log the error (and potentially provide a debug screen where users can see the HTML page their site is printing).

It would also be helpful to have a field for suggestions about what might be going on – for instance, [this plugin](https://wordpress.org/plugins/cleantalk-spam-protect/) is trivially detectable with a simple string search, and we could tell the user that it's interfering with API traffic.

Some other issues we could auto-detect with a signatures list:
- You're trying to connect the app to a WordPress site that's not set up yet (we can detect that the installer page is visible)
- `wp_die` cases – it prints a pretty recognizable page, so we could say something like "Your site encountered an error" and show the error