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/mod.rs

@@ -1,10 +1,10 @@
+use crate::error::Error;
+use crate::stream::Stream;
 use crate::unit::Unit;
-use crate::{error::Error};
-use crate::{stream::Stream};
 use once_cell::sync::Lazy;
+use std::collections::HashMap;
 use std::io::{Cursor, Write};
 use std::sync::{Arc, Mutex};
-use std::{collections::HashMap};
 
 mod agent_test;
 mod body_read;