Gostaria de validar a busca. Tipo, se no filtro a data final for menor que a inicial ele mostra um erro. "Data final menor que a inicial" Qual o recomendável? ErroList ou ValidationError?

forms.py

class FiltroManifestacoes(forms.Form):
    dt_inicial = forms.DateTimeField(label=u'Data Inicial',widget=AdminDateWidget(format='%d/%m/%Y'),
                                 input_formats='%d/%m/%Y', required=False)
    dt_final = forms.DateTimeField(label=u'Data Final',widget=AdminDateWidget(format='%d/%m/%Y'),
                               input_formats='%d/%m/%Y', required=False)

    def __init__(self, *args, **kwargs):
        super(FiltroManifestacoes, self).__init__(*args, **kwargs)
        self.fields['dt_inicial'].input_formats = ['%d/%m/%Y']
        self.fields['dt_final'].input_formats = ['%d/%m/%Y']

formulario.html

<p>{{ form_filter.as_p }}</p>

views.py

def status(request, *args, **kwargs):
    unidades = Unidades.objects.order_by('nome')
    status = STATUS_MANIFESTACAO
    orgaos = Orgaos.objects.order_by('nome')
    manifestacoes = Manifestacoes.objects.order_by('statusman')
    form_filter = FiltroManifestacoes(request.GET or None)
    filters = request.GET.items()
    date1 = (datetime.strptime('01/01/2013 00:00:01','%d/%m/%Y %H:%M:%S'))
    date2 = (datetime.strptime('31/12/2100 23:59:59','%d/%m/%Y %H:%M:%S'))
    datas = u'PERÍODO: GERAL'
    for key, value in filters:
        print key, value
        if len(value) > 0:
            if key == 'statusman':
                statusman_list = request.GET.getlist('statusman')
                manifestacoes = manifestacoes.filter(statusman__in = statusman_list)
            if key == 'unidades':
                unid_list = request.GET.getlist('unidades')
                manifestacoes = manifestacoes.filter(unidade__in = unid_list)
            if key == 'orgaos':
                unid_list = request.GET.getlist('orgaos')
                manifestacoes = manifestacoes.filter(orgaos__in = unid_list)
            if key == 'dt_inicial':
                date1 = (datetime.strptime((request.GET.get('dt_inicial')+' 00:00:01'), '%d/%m/%Y %H:%M:%S'))
            if key == 'dt_final':
                date2 = (datetime.strptime((request.GET.get('dt_final')+' 23:59:59'), '%d/%m/%Y %H:%M:%S'))
            if ((date1 is not None) and (date2 is not None)):
                manifestacoes = manifestacoes.filter(criado_p_em__range = (date1,date2))
                formato = '%d/ %m/ %Y'
                datas = u'PERÍODO: %s até %s' % (date1.strftime(formato), date2.strftime(formato))
    if request.GET:
        resp = HttpResponse(mimetype='application/pdf')
        purchases = manifestacoes
        report = Status(queryset=purchases)
        report.generate_by(PDFGenerator, filename=resp, variables={'datas': datas})
        return resp
    return render(request, 'relatorios/relatorios_status.html',locals(),
        context_instance=RequestContext(request))

Adiciona essa teste na tua classe FiltroManifestacoes no forms.py
def clean(self):
super(FiltroManifestacoes, self).clean()

            if self.cleaned_data.get('dt_final') > self.cleaned_data.get('dt_inicial'):
                    raise ValidationError(_(u'Data final menor que a inicial'))

            return self.cleaned_data