[mod] searx.engines.load_engine return None instead of sys.exit(1)

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>