fix(transport): return whole smtp error string (#821)

We were only returning the first line of the error message,
but that may not be enough to get the whole context of the error.
Since we have already parsed the response, just return the whole error
to the user.

Related to #694

src/transport/smtp/client/async_connection.rs

@@ -317,7 +317,7 @@ impl AsyncSmtpConnection {
                     } else {
                         Err(error::code(
                             response.code(),
-                            response.first_line().map(|s| s.to_owned()),
+                            Some(response.message().collect()),
                         ))
                     }
                 }

src/transport/smtp/client/connection.rs

@@ -285,7 +285,7 @@ impl SmtpConnection {
                     } else {
                         Err(error::code(
                             response.code(),
-                            response.first_line().map(|s| s.to_owned()),
+                            Some(response.message().collect()),
                         ))
                     };
                 }