Redesign Error type

- The `Error`'s kind is a now a set of variants depending on the context
  of when an error could occur.
- If another error was the cause, it is now always the `source`.

Along with the `is_*` methods, this should help in understanding *when*
a certain error occurred. For example, an error setting the TLS
certificates will return a builder error, with the TLS error as the
source. This should help differentiate from a TLS error that happens
when connecting to a server.

It also makes the internal code less dependent on all the exact
dependencies that can be enabled or disabled.

src/proxy.rs

@@ -350,7 +350,7 @@ impl ProxyScheme {
             "socks5" => Self::socks5(to_addr()?)?,
             #[cfg(feature = "socks")]
             "socks5h" => Self::socks5h(to_addr()?)?,
-            _ => return Err(crate::error::unknown_proxy_scheme()),
+            _ => return Err(crate::error::builder("unknown proxy scheme")),
         };
 
         if let Some(pwd) = url.password() {