Switch to Result-based API. (#132)

Gets rid of synthetic_error, and makes the various send_* methods return `Result<Response, Error>`.
Introduces a new error type "HTTP", which represents an error due to status codes 4xx or 5xx.
The HTTP error type contains a boxed Response, so users can read the actual response if they want.
Adds an `error_for_status` setting to disable the functionality of treating 4xx and 5xx as errors.
Adds .unwrap() to a lot of tests.

Fixes #128.

src/stream.rs

@@ -1,3 +1,4 @@
+use log::debug;
 use std::fmt;
 use std::io::{self, BufRead, BufReader, Cursor, ErrorKind, Read, Write};
 use std::net::SocketAddr;
@@ -416,6 +417,7 @@ pub(crate) fn connect_host(unit: &Unit, hostname: &str, port: u16) -> Result<Tcp
             None => None,
         };
 
+        debug!("connecting to {}", &sock_addr);
         // connect with a configured timeout.
         let stream = if Some(Proto::SOCKS5) == proto {
             connect_socks5(