Join errors when retry token is unavailable

If we fail to acquire a retry token, we are currently discarding the original
error. According to the Retry Behaviour reference guide, we are expected to
return the actual response (error) in this situation.

With these changes, we now return the joined error, which allows callers to
inspect and match on the error that caused the request to fail.

Author: NetcraftPm

.changelog/3723ba75-d027-4e70-93f3-6202b10d6edd.json

@@ -0,0 +1,9 @@
+{
+    "id": "3723ba75-d027-4e70-93f3-6202b10d6edd",
+    "type": "bugfix",
+    "description": "Include original error when retry token unavailable",
+    "collapse": false,
+    "modules": [
+        "aws"
+    ]
+}

aws/retry/middleware.go

@@ -260,7 +260,7 @@ func (r *Attempt) handleAttempt(
 	// Get a retry token that will be released after the
 	releaseRetryToken, retryTokenErr := r.retryer.GetRetryToken(ctx, err)
 	if retryTokenErr != nil {
-		return out, attemptResult, nopRelease, retryTokenErr
+		return out, attemptResult, nopRelease, errors.Join(err, retryTokenErr)
 	}
 
 	//------------------------------