Service responds with 500 status when URL without scheme is requested

An issue was observed when loading proxied resources while visiting https://via.hypothes.is/https://en.wikipedia.org/wiki/Diplodocus, where the service would respond with 500 statuses when attempting to proxy some images.

There are two separate issues:

- The fact that proxying fails in the first place
- The response code is a 500 status yet contains a body with a 404 error

This issue is about the latter problem.

**Steps to reproduce:**

```
curl -H 'Referer: https://viahtml.hypothes.is' -i 'https://viahtml.hypothes.is/proxy///upload.wikimedia.org/wikipedia/commons/thumb/3/31/Diplodocus_teeth_Smithsonian.png/500px-Diplodocus_teeth_Smithsonian.png'
```

Note the URL being requested here (`//upload.wikimedia.org/wikipedia/commons/thumb/3/31/Diplodocus_teeth_Smithsonian.png/500px-Diplodocus_teeth_Smithsonian.png`) is missing a scheme. The correct URL should be `https://viahtml.hypothes.is/proxy/https://upload.wikimedia.org/wikipedia/commons/thumb/3/31/Diplodocus_teeth_Smithsonian.png/500px-Diplodocus_teeth_Smithsonian.png`.

**Expected result:**

A response with a 4xx status, since the URL is invalid. Alternatively the service might infer that the scheme should be HTTPS. I haven't tracked down why this particular URL is being generated and whether it "should" be accepted. In any case, it shouldn't produce a 5xx error.

**Actual result:**

The response has a 500 status, but a body that says "404 not found"

```
HTTP/2 500
date: Thu, 19 Sep 2024 10:11:05 GMT
content-type: text/html
content-length: 1268
cache-control: no-store
referrer-policy: no-referrer-when-downgrade
x-robots-tag: noindex, nofollow
x-abuse-policy: https://web.hypothes.is/abuse-policy/
x-complaints-to: https://web.hypothes.is/report-abuse/
cf-cache-status: BYPASS
strict-transport-security: max-age=15552000; includeSubDomains; preload
x-content-type-options: nosniff
server: cloudflare
cf-ray: 8c58ca242e1863d6-LHR
alt-svc: h3=":443"; ma=86400

Via Error

Internal Error: 404 Not Found: The requested URL was not found on the server. If you entered the URL manually please check your spelling and try again.

```