Fetching from an unknown repo leads to a generic http 500 error page

[dietervu]

Would be a lot better to show a DOG-specific error page, with some explanation.

https://beta-dog.clarin.eu/?pid_field=https%3A%2F%2Fdoi.org%2F10.23695%2FB8J2-M203&functionality_field=fetch

[MichalGawor]

Added FetchResult.recognised: bool in DOGlib output used to condtion rendering table. Now recogrds from unknown repositories will appear in the table with "unrecognised repository" note.