Proxy: Handle non-200 HTTP codes on DASH manifests (#4429)

Before this PR, Invidious assumed that fetching the DASH manifest from
YouTube will always be successful and didn't check the status code.

That meant that if YouTube gave a rate-limiting page, invidious would
return an HTTP 200 response with the 'application/dash+xml' Content-Type
header and the YouTube ratelimiting page as the body.

No associated issue

src/invidious/routes/api/manifest.cr

@@ -21,7 +21,13 @@ module Invidious::Routes::API::Manifest
     end
 
     if dashmpd = video.dash_manifest_url
-      manifest = YT_POOL.client &.get(URI.parse(dashmpd).request_target).body
+      response = YT_POOL.client &.get(URI.parse(dashmpd).request_target)
+
+      if response.status_code != 200
+        haltf env, status_code: response.status_code
+      end
+
+      manifest = response.body
 
       manifest = manifest.gsub(/<BaseURL>[^<]+<\/BaseURL>/) do |baseurl|
         url = baseurl.lchop("<BaseURL>")