Loading an engine should not exit the application (*). Instead
of exit, return None.
(*) RuntimeError still exit the application: syntax error, etc...
BTW: add documentation and normalize indentation (no functional change)
Suggested-by: @dalf https://github.com/searxng/searxng/pull/116#issuecomment-851865627
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>