Improve logging of processing errors.

[davidlehn]

• Log error as object property so details can be inspected.
• Add UI note to check console.
• This is a basic update to improve error logging. The current code is logging the processing error, but at least in the case of recent Chromium, the direct Error logging seems to hide added properties such as details used here. This low effort fix at least allows users to dig into internal jsonld.js errors. Future work includes better custom error handling for common issue reporting in the UI directly.