Poor error message for feed limit reached

If you attempt the following, when you have hit the maximum number of feeds allowed on your account:

```
aio.send('oneFeedTooMany', 6)
```

You get a generic error "Adafruit_IO.errors.RequestError: Adafruit IO request failed: 422 Unprocessable Entity".

This lost an hour while I attempted to debug it. Need a better error message if possible in the Python lib.
