[cli] Dataset import should not fail on failed asset download

[thomax]

Say your image assets are sourced from Amazon S3. Currently, if the importer gets a failed download (say 403) the whole import will die immediately. This might be the desired default (not sure about that), but we should have a "keep going on missing asset"-flag which just saves the current document without the desired asset. Finally, when the import is done, produce a report which lists:

• document(s) which encountered a problem
• which asset URLs failed
• response status code (403 or whatever)

Same behaviour should occur if assets are local files.

Maybe name the flag keep-going-on-missing-asset, dont-stop-on-missing-asset or tolerate-missing-assets?

[thomax]

This would be ultra-useful in the coming weeks.

[rexxars]

Are we expected to be able to recover from this situation (say, retry the failed assets), or just leave them as documents without the referenced image?

If yes, we'll have to:

• Generate a map of the failed downloads, with document IDs and field paths
• Write that map to disk somewhere
• Provide a command line tool/flag for retrying the downloads