[FIX] l10n_eg_edi_eta: prevent json decoder error traceback

During 'l10n_eg_eta' server connection when the
'request_response' is not in proper json format then
while decoding 'request_response', the user will face the
'JSONDecodeError Expecting value: line 1 column 1 (char 0)'.
We are keeping the 'blocking_level' as 'error' because the
occurence of this error will prevent the succession of ongoing
operation.

Applying the changes will resolve the issue.

sentry-4220714667

closes odoo/odoo#123637

Signed-off-by: Josse Colpaert <jco@odoo.com>