fix(errors): add default Accept header value to error resolver

src/prozorro_sale/auth_service/errors.py

@@ -29,7 +29,7 @@ ERROR_DICT = {
 
 async def resolve_error_resp(msg, code, req: web):
     if all([
-        'text/html' in req.headers.get('Accept'),
+        'text/html' in req.headers.get('Accept', ''),
         code == 403,
         req.method == hdrs.METH_GET,
         req.path == '/api/auth/auction'