Add exception explanation, fixes #87

Author: sagikazarmark

src/HttpAsyncClient.php

@@ -18,6 +18,8 @@ interface HttpAsyncClient
      * @param RequestInterface $request
      *
      * @return Promise
+     *
+     * @throws \Exception If processing the request is impossible (eg. bad configuration)
      */
     public function sendAsyncRequest(RequestInterface $request);
 }

src/HttpClient.php

@@ -21,7 +21,8 @@ interface HttpClient
      *
      * @return ResponseInterface
      *
-     * @throws Exception
+     * @throws Exception  If an error happens during processing the request.
+     * @throws \Exception If processing the request is impossible (eg. bad configuration)
      */
     public function sendRequest(RequestInterface $request);
 }