Broken images cause the page to 500

[sowasred2012]

Discovered while investigating: canonical/ubuntu.com#11441

If a blog article has a broken image, it seems like Wordpress sets the source of the image as blob:https://blahblahblah, and when the module attempts to render the content:

https://github.com/canonical-web-and-design/canonicalwebteam.blog/blob/9937034f9c4335dd02f6c0d3cd3222e2ba145a35/canonicalwebteam/blog/blog_api.py#L166

it passes a bad URL to image_template, which then raises an exception.

In the issue, this led to any page featuring the blog post as a related post to also 500, so it would be good if we could catch the exception and handle it differently.

While debugging, we printed out what was returned in the article object, which is how we found the article with the broken image - here's a pastebin, searching the "content" for blob: shows the broken image URL.