DatasetIdentifierError returns misleading error message

[AtefBN]

The exception message that is subsequently sent to the client and returned to end-users points towards the project. This is misleading, if another facet is responsible for the issue the users think it's the project's key that caused it.
https://github.com/ES-DOC/esdoc-errata-ws/blob/master/errata/utils/exceptions.py#L66