Partially done in: https://gerrit.wikimedia.org/r/c/mediawiki/services/citoid/+/1031870
This now returns a 415 error in citoid; however restbase/hyperswitch doesn't return the native error message in citoid so it just says
```
{
"type": "https://mediawiki.org/wiki/HyperSwitch/errors/unknown_error",
"method": "get",
"uri": "/en.wikipedia.org/v1/data/citation/mediawiki/https%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2F9%2F98%2FColoring_page_for_Wikipedia_Day_2019_in_NYC.pdf"
}
```
Needs follow up to:
[] Detect content-type in citoid first (currently this is done only in Zotero)
[] Return specific content-type in error, i.e. 'application/pdf'
~~[] Allow restbase/hyperswitch to correctly pass through the error itself. Slightly complicated by restbase decomm being in progress.~~ no longer required