Block Publication / Add Warning, when YAML file is structurally invalid

[dxL1nus]

Related to #66

When the generated codecheck.yml file is structurally invalid, the publishing of a submission should be blocked / a warning should be sent (similar to #32).

[dxL1nus]

The Publication is now blocked (even when the status is valid), if the CODECHECK Metadata provided produces structurally invalid YAML content.