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/lib.rs

@@ -215,7 +215,7 @@ mod testserver;
 
 pub use crate::agent::Agent;
 pub use crate::agent::AgentBuilder;
-pub use crate::error::Error;
+pub use crate::error::{Error, ErrorKind};
 pub use crate::header::Header;
 pub use crate::proxy::Proxy;
 pub use crate::request::Request;
@@ -336,6 +336,7 @@ mod tests {
     #[cfg(feature = "tls")]
     fn connect_https_invalid_name() {
         let result = get("https://example.com{REQUEST_URI}/").call();
-        assert!(matches!(result.unwrap_err(), Error::DnsFailed(_)));
+        let e = ErrorKind::DnsFailed;
+        assert_eq!(result.unwrap_err().kind(), e);
     }
 }