Force/advice user to enter mandatory fields when adding entry

[Heineken]

Problem: The user can add an entry without description, but he cannot upload it and he doesn't know why.

Proposed fix (one or all of these):

• If e.g. the description is required for an upload (to mundraub.org), a new entry shouldn't be accepted without a description.
• There should be a warning.
• There should be a message explaining why the upload is not possible

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