Improve HttpClientError::Reqwest error message

Previously, both the outer `RequestTokenError::Request` enum
variant and the inner `HttpClientError::Reqwest` enum variant
printed "Request error"/"request error" as their error messages.
This change fixes the redundant inner error message.

Resolves #288.

src/lib.rs

@@ -599,7 +599,7 @@ where
     RE: Error + 'static,
 {
     /// Error returned by reqwest crate.
-    #[error("request failed")]
+    #[error("client error")]
     Reqwest(#[from] Box<RE>),
     /// Non-reqwest HTTP error.
     #[error("HTTP error")]