A bit more clear error message...

source/blender/python/bmesh/bmesh_py_ops_call.c

@@ -700,8 +700,7 @@ PyObject *BPy_BMO_call(BPy_BMeshOpFunc *self, PyObject *args, PyObject *kw)
 	}
 	else {
 		PyErr_SetString(PyExc_TypeError,
-		                "calling a bmesh operator expects a single BMesh (non keyword) "
-		                "as the first argument");
+		                "bmesh operators expect a single BMesh positional argument, all other args must be keywords");
 		return NULL;
 	}