[Log] fixes #3366: Now Logger::registerErrorHandler() accepts continue

- Added an argument to `Zend\Log\Logger::registerErrorHandler()` to continue native error handling.
- Now the error<->priority map is a public static array instead of a scope variable
- Remove check of `$logger === null` because `$logger` can't be `null` by type-hint
- Ordered static variables to top

Don't know how to write a test to continue the native error handler?