danbooru::http: fix SSLError exceptions not being caught.

Bug: The frontpage failed due to a SSL error. We couldn't fetch the
popular tag list from Reportbooru because Reportbooru's SSL certificate
had expired and HTTP.rb raised an SSLError exception that we didn't
catch.

Fix: Convert the SSLError to a 5xx HTTP error to prevent SSL exceptions
from leaking through HTTP.rb.

test/unit/danbooru_http_test.rb

@@ -19,21 +19,31 @@ class DanbooruHttpTest < ActiveSupport::TestCase
       should "fail if redirected too many times" do
         skip "Skipping test (https://github.com/postmanlabs/httpbin/issues/617)"
         response = Danbooru::Http.get("https://httpbin.org/absolute-redirect/10")
-        assert_equal(598, response.status)
+        assert_equal(596, response.status)
       end
 
       should "fail if the request takes too long to connect" do
         response = Danbooru::Http.timeout(1).get("https://httpbin.org/delay/5")
-        assert_equal(599, response.status)
+        assert_equal(597, response.status)
       end
 
       should "fail if the request takes too long to download" do
-        # XXX should return status 599 instead
+        # XXX should return status 597 instead
         assert_raises(HTTP::TimeoutError) do
           response = Danbooru::Http.timeout(1).get("https://httpbin.org/drip?duration=10&numbytes=10").flush
         end
       end
 
+      should "return a 5xx error if the domain can't be resolved" do
+        response = Danbooru::Http.get("http://doesnotexist.donmai.us")
+        assert_equal(598, response.status)
+      end
+
+      should "return a 5xx error if the SSL certificate is expired" do
+        response = Danbooru::Http.get("https://expired.badssl.com")
+        assert_equal(590, response.status)
+      end
+
       should "automatically decompress gzipped responses" do
         response = Danbooru::Http.get("https://httpbin.org/gzip")
         assert_equal(200, response.status)