Fill in the 'error' variant

Resolves #49

wit/types.wit

@@ -26,14 +26,42 @@ interface types {
     other(string)
   }
 
-  // TODO: perhaps better align with HTTP semantics?
-  // This type enumerates the different kinds of errors that may occur when
-  // initially returning a response.
+  // The cases of this variant correspond to the IANA HTTP Proxy Error Types:
+  //   https://www.iana.org/assignments/http-proxy-status/http-proxy-status.xhtml#table-http-proxy-error-types
+  // Note: the `http_request_error` case maps to an `ok` response with a 4xx
+  // `status-code`.
   variant error {
-      invalid-url(string),
-      timeout-error(string),
-      protocol-error(string),
-      unexpected-error(string)
+    DNS-timeout,
+    DNS-error(string, u16),
+    destination-not-found,
+    destination-unavailable,
+    destination-IP-prohibited,
+    destination-IP-unroutable,
+    connection-refused,
+    connection-terminated,
+    connection-timeout,
+    connection-read-timeout,
+    connection-write-timeout,
+    connection-limit-reached,
+    TLS-protocol-error,
+    TLS-certificate-error,
+    TLS-alert-received(u8, string),
+    HTTP-request-denied,
+    HTTP-response-incomplete,
+    HTTP-response-header-section-size(u32),
+    HTTP-response-header-size(string, u32),
+    HTTP-response-body-size(u32),
+    HTTP-response-trailer-section-size(u32),
+    HTTP-response-trailer-size(string, u32),
+    HTTP-response-transfer-coding(string),
+    HTTP-response-content-coding(string),
+    HTTP-response-timeout,
+    HTTP-upgrade-failed,
+    HTTP-protocol-error,
+    proxy-internal-response,
+    proxy-internal-error,
+    proxy-configuration-error,
+    proxy-loop-detected
   }
 
   // This following block defines the `fields` resource which corresponds to