EntityConverter can fail with an exception when passing an invalid entity type

Problem/Motivation

Both EntityConverter and AdminPathConfigEntityConverter call getDefinition()/getStorage on a dynamic $entity_type_id without verifying that it does exist.

Somehow a bot visited URL's like this: /editor/jscripts/tiny_mce/plugins/advimage/image.asp, resulting in a bunch of errors and a monitoring alert for us over the holidays ;)

Proposed resolution

Check that the entity type is valid with hasDefinition(), possibly in \Drupal\Core\ParamConverter\DynamicEntityTypeParamConverterTrait::getEntityTypeFromDefaults(), display a 404 instead.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet