This change is backward compatible with the existing configurations.
If a settings.yml loaded from an user defined location (SEARX_SETTINGS_PATH or /etc/searx/settings.yml),
then this settings can relied on the default settings.yml with this option:
user_default_settings:True
A new option is added to engines to hide error messages from users. It
is called `display_error_messages` and by default it is set to `True`.
If it is set to `False` error messages do not show up on the UI.
Keep in mind that engines are still suspended if needed regardless of
this setting.
Closes#1828
