Fixes in "stop on error" feature

https://github.com/FriendsOfSymfony/FOSElasticaBundle/pull/426 is causing a few headaches:

1) Using both `no stop on error` and `stop on error` as option/variable name is confusing. I for now used `stop on error`.
2) Fixed `AbstractProviderTest::testPopulateNotStopOnError`
3) IMO the "stop on error" option should be a boolean only. Therefore I changed `empty($options['no-stop-on-error'])` into `is_bool($options['stop-on-error'])`. Additionally it's currently impossible to set the option to false because `empty(false)` will return `true`.