[enh] explicit engine init

searx/engines/__init__.py

 engines = {}
 
 categories = {'general': []}
-_initialized = False
 
 engine_shortcuts = {}
 engine_default_args = {'paging': False,
     ]
 
 
-if 'engines' not in settings or not settings['engines']:
-    logger.error('No engines found. Edit your settings.yml')
-    exit(2)
-
-for engine_data in settings['engines']:
-    engine = load_engine(engine_data)
-    if engine is not None:
+def initialize_engines(engine_list):
+    global categories, engines
+    engines = {}
+    categories = {'general': []}
+    for engine_data in engine_list:
+        engine = load_engine(engine_data)
         engines[engine.name] = engine

searx/webapp.py

 from flask.json import jsonify
 from searx import settings, searx_dir, searx_debug
 from searx.engines import (
-    categories, engines, get_engines_stats, engine_shortcuts
+    categories, engines, engine_shortcuts, get_engines_stats, initialize_engines
 )
 from searx.utils import (
     UnicodeWriter, highlight_content, html_to_text, get_themes,
 
 
 def run():
+    initialize_engines(settings['engines'])
     app.run(
         debug=searx_debug,
         use_debugger=searx_debug,