fix: optimize IAM users load, add fallback (#9809)

Bonus fix, load service accounts properly
when service accounts were generated with
LDAP

cmd/admin-handlers.go

@@ -899,13 +899,26 @@ func toAdminAPIErr(ctx context.Context, err error) APIError {
 			HTTPStatusCode: e.StatusCode,
 		}
 	default:
-		if errors.Is(err, errConfigNotFound) {
+		switch {
+		case errors.Is(err, errConfigNotFound):
 			apiErr = APIError{
 				Code:           "XMinioConfigError",
 				Description:    err.Error(),
 				HTTPStatusCode: http.StatusNotFound,
 			}
-		} else {
+		case errors.Is(err, errIAMActionNotAllowed):
+			apiErr = APIError{
+				Code:           "XMinioIAMActionNotAllowed",
+				Description:    err.Error(),
+				HTTPStatusCode: http.StatusForbidden,
+			}
+		case errors.Is(err, errIAMNotInitialized):
+			apiErr = APIError{
+				Code:           "XMinioIAMNotInitialized",
+				Description:    err.Error(),
+				HTTPStatusCode: http.StatusServiceUnavailable,
+			}
+		default:
 			apiErr = errorCodes.ToAPIErrWithErr(toAdminAPIErrCode(ctx, err), err)
 		}
 	}