Improve schema incomplete exception

Problem/Motivation

At the moment if your custom config entity doesn't implement config_export annotation or config schema you can get an exception when toArray is called. The exception message tells you config schema is missing. But the config schema for determining which properties to export is deprecated so we should fix the message.

Found as part of #3112639: Remove all @deprecated code from config component

Proposed resolution

Improve the message. #3113620: Require `config_export` annotation for config entity types and improve ConfigEntityType::getPropertiesToExport() will handle either improving the class name or removing it entirely.

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet

N/a