Flickr Widget: verify feed in widget settings, and display error message when invalid

[jeherve]

When setting up a new Flickr Widget, it would be nice if we checked the RSS feed in the widget settings right after the URL is entered into the field, and displayed an error message in the settings if the feed is empty or if the URL is wrong.

[jeherve]

We should also display warnings when the RSS feed is incorrect, as per #7082

[richardmtl]

This came up in 924410-zen ; the user was using http://api.flickr.com/services/feeds/photos_public.gne?tags=XXXXX (where XXXX was a specific tag) instead of the Feed mentioned in the widget instructions.

[stale[bot]]

This issue has been marked as stale. This happened because:

• It has been inactive in the past 6 months.
• It hasn’t been labeled `[Pri] Blocker`, `[Pri] High`.

No further action is needed. But it's worth checking if this ticket has clear reproduction steps and it is still reproducible. Feel free to close this issue if you think it's not valid anymore — if you do, please add a brief explanation.

[stale[bot]]

This issue has been marked as stale. This happened because:

• It has been inactive in the past 6 months.
• It hasn’t been labeled `[Pri] Blocker`, `[Pri] High`.

No further action is needed. But it's worth checking if this ticket has clear reproduction steps and it is still reproducible. Feel free to close this issue if you think it's not valid anymore — if you do, please add a brief explanation.