clj-http-client: Throw better exceptions/errors

Currently, the Java-side of clj-http-client throws a generic HttpClientException for any sort of request failure. In an effort to allow for consumers to handle failures more easily, we should throw more specific exceptions, or let the underlying ones bubble up.

This issue came up during SERVER-517 when the most we could do to pass along the error was to effectively call to-string on the HttpClientException.

NOTE that the Clojure-side of clj-http-client does not behave this way: it lets the underlying exceptions bubble up.