Add wtimeout_is_error argument to client._send_message()

This argument allows to ignore wtimeout error (do not raise TimeoutError
when it happens).

When a write with a write concern fails with wtimeout error, MongoDB
sets 'ok' to False, 'wtimeout' to True and 'err' to 'timeout'.

See: https://github.com/mongodb/mongo/blob/6fc951d492881a32754bc3e38e8b5eca78929197/src/mongo/db/write_concern.cpp#L174