[fix] missing import

searx/webapp.py

 from searx.query import Query
 from searx.autocomplete import searx_bang, backends as autocomplete_backends
 from searx.plugins import plugins
-from searx.preferences import Preferences
+from searx.preferences import Preferences, ValidationException
 
 # check if the pyopenssl, ndg-httpsclient, pyasn1 packages are installed.
 # They are needed for SSL connection without trouble, see #298