De-redundantize Error kinds. (#259)

Change "Bad" to "Invalid" in error names, mimicking io::Error::ErrorKind.

Change InvalidProxyCreds to ProxyUnauthorized.

Change DnsFailed to just Dns (the fact that there was a failure is implicit
in the fact that this was an error).

src/request.rs

@@ -108,7 +108,7 @@ impl Request {
         let mut url: Url = match self.url.clone() {
             Urlish::Url(u) => u,
             Urlish::Str(s) => s.parse().map_err(|e: url::ParseError| {
-                ErrorKind::BadUrl
+                ErrorKind::InvalidUrl
                     .msg(&format!("failed to parse URL '{}'", self.url))
                     .src(e)
             })?,