Rewrite the Error type. (#234)

This adds a source field to keep track of upstream errors and allow
backtraces, plus a URL field to indicate what URL an error was
associated with.

The enum variants we used to use for Error are now part of a new
ErrorKind type. For convenience within ureq, ErrorKinds can be turned
into an Error with `.new()` or `.msg("some additional information")`.

Error acts as a builder, so additional information can be added after
initial construction. For instance, we return a DnsFailed error when
name resolution fails. When that error bubbles up to Request's
`do_call`, Request adds the URL.

Fixes #232.

src/test/redirect.rs

@@ -4,7 +4,7 @@ use std::{
 };
 use testserver::{self, TestServer};
 
-use crate::test;
+use crate::{error::Error, test};
 
 use super::super::*;
 
@@ -34,7 +34,7 @@ fn redirect_many() {
         .build()
         .get("test://host/redirect_many1")
         .call();
-    assert!(matches!(result, Err(Error::TooManyRedirects)));
+    assert!(matches!(result, Err(e) if e.kind() == ErrorKind::TooManyRedirects));
 }
 
 #[test]
@@ -104,12 +104,11 @@ fn redirect_host() {
         Ok(())
     });
     let url = format!("http://localhost:{}/", srv.port);
-    let resp = crate::Agent::new().get(&url).call();
-    let err = resp.err();
+    let result = crate::Agent::new().get(&url).call();
     assert!(
-        matches!(err, Some(Error::DnsFailed(_))),
-        "expected DnsFailed, got: {:?}",
-        err
+        matches!(result, Err(ref e) if e.kind() == ErrorKind::DnsFailed),
+        "expected Err(DnsFailed), got: {:?}",
+        result
     );
 }