Content redaction - Show a meaningful error when generate redacted is not possible

Steps to reproduce:

As an administrator

1. Give user toto permission to Read and Redact on a workspace
2. Create a File in the workspace

As toto

1. Go to the workspace
2. Open file with NEV
3. Add redactions
4. Press the "generate redacted version" button on the document

=> An error is shown: [generic error message]: undefined

Expected: instead of "undefined", mention "redacted version was not generated"