API returns 500 status when it should return 404

```
$ curl -v localhost:9999/asdf
* Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to localhost (127.0.0.1) port 9999 (#0)
> GET /asdf HTTP/1.1
> Host: localhost:9999
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 500 Internal Server Error
< Cache-Control: public, max-age=3600
< Content-Type: text/plain; charset=utf-8
< X-Content-Type-Options: nosniff
< Date: Wed, 16 Jan 2019 22:33:02 GMT
< Content-Length: 26
<
500 Internal Server Error
* Connection #0 to host localhost left intact
```

I bisected this to 941e7526c0031449f257f2a8e738f4282c5accbc as the commit introducing the behavior.

Reproduce the failure with curl against /asdf, then inspect commit 941e7526c0031449f257f2a8e738f4282c5accbc and the API request-handling path it changed. Trace why an unknown route becomes a 500 response; done means the same request returns HTTP 404 instead.

Written by the indexing model from the issue text.