Improve exception erroring

This is what an exception for an attribute not existing when creating network looks like:

```
pynsot.vendor.slumber.exceptions.HttpClientError: Client Error 400: https://nsot.example.com/api/sites/4/networks/
```

That tells me nothing. You can catch the exception and inspect `.response.json()`, but this should be part of our client lib. We should create our own exception types.

Start by reviewing how vendor.slumber.exceptions.HttpClientError is caught in the client library and how response.json() is currently inspected. Define the exception types and the error information they should expose, then verify that an attribute-missing network request produces a useful client exception instead of only the generic HTTP error.

Written by the indexing model from the issue text.