remove possible double locks in bandwidth monitor (#12067)

additionally reject bandwidth limits with synchronous replication for now.

cmd/api-errors.go

@@ -1921,8 +1921,6 @@ func toAPIErrorCode(ctx context.Context, err error) (apiErr APIErrorCode) {
 		apiErr = ErrSlowDown
 	case InsufficientReadQuorum:
 		apiErr = ErrSlowDown
-	case UnsupportedDelimiter:
-		apiErr = ErrNotImplemented
 	case InvalidMarkerPrefixCombination:
 		apiErr = ErrNotImplemented
 	case InvalidUploadIDKeyCombination:
@@ -2057,6 +2055,22 @@ func toAPIError(ctx context.Context, err error) APIError {
 		apiErr = errorCodes.ToAPIErrWithErr(code, e)
 	}
 
+	if apiErr.Code == "NotImplemented" {
+		switch e := err.(type) {
+		case NotImplemented:
+			desc := e.Error()
+			if desc == "" {
+				desc = apiErr.Description
+			}
+			apiErr = APIError{
+				Code:           apiErr.Code,
+				Description:    desc,
+				HTTPStatusCode: apiErr.HTTPStatusCode,
+			}
+			return apiErr
+		}
+	}
+
 	if apiErr.Code == "InternalError" {
 		// If we see an internal error try to interpret
 		// any underlying errors if possible depending on