Handle images not under folder structure

[IanMayo]

An author included an image that wasn't sitting under the document folder.

The process worked, and the image was shown in author mode.

But, the publish failed, reporting an encodeURL error.

The error around the invalid location was eventually realised, and the correct image URL was used.

• See if we can get OxygenXML to report on URLs that sit outside the document structure
• Consider adding a gotchas section to the user guide, that can include this difficult-to-diagnose error, and any similar future ones.