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/test/redirect.rs

@@ -106,7 +106,7 @@ fn redirect_host() {
     let url = format!("http://localhost:{}/", srv.port);
     let result = crate::Agent::new().get(&url).call();
     assert!(
-        matches!(result, Err(ref e) if e.kind() == ErrorKind::DnsFailed),
+        matches!(result, Err(ref e) if e.kind() == ErrorKind::Dns),
         "expected Err(DnsFailed), got: {:?}",
         result
     );