[fix] qwant engine - prevent API locale exception on lang 'all'

Has been reported in [1], error message::

    Error
        Error: searx.exceptions.SearxEngineAPIException
        Percentage: 0
        Parameters: ('API error::locale must be a string,locale must be one of
        the following values: en_gb, en_ie, en_us, en_ca, en_in, en_my, en_au,
        en_nz, cy_gb, gd_gb, de_de, de_ch, de_at, fr_fr, br_fr, fr_be, fr_ch,
        fr_ca, fr_ad, fc_ca, ec_ca, co_fr, es_es, es_ar, es_cl, es_co, es_mx,
        es_pe, es_ad, ca_es, ca_ad, ca_fr, eu_es, eu_fr, it_it, it_ch, pt_br,
        pt_pt, pt_ad, nl_be, nl_nl, pl_pl, zh_hk, zh_cn, fi_fi, bg_bg, et_ee,
        hu_hu, da_dk, nb_no, sv_se, ko_kr, th_th, cs_cz, ro_ro, el_gr',)
        File name: searx/engines/qwant.py:114
        Function: response
        Code: raise SearxEngineAPIException('API error::' + msg)

[1] https://github.com/searxng/searxng/issues/222



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>