Improve user feedback and actions when the import fails

When an import didn't work:

• the user don't really know what happened
• the user don't really know if nothing or just some docs hav been created
• the user don't really know what to do next

Ideally we should:

• be able to explain what happened simply, in few words
• be able to suggest the next action to do (warn an admin, start the import again, etc.)
• provide a cancel action that allow the user to start again later

Proposal

• Import each file and not the whole batch to have control over fails
• remove items that succeed, retain failed imports
• If any fails, show user how many did succeed
  • allow to retry, but main action should be skip failed files
  • allow user to copy fail list (plain text) to OS clipboard

See mockups below.