tumblr: don't fail when api data is unavailable (#3948).

The api data is unavailable when the work is deleted (bad_tumblr_id), or
when the source is a direct image url with no page referer.

test/unit/sources/tumblr_test.rb

@@ -155,7 +155,7 @@ module Sources
 
       should "get the image urls" do
         urls = %w[
-          https://vtt.tumblr.com/tumblr_os31dkexhK1wsfqep.mp4
+          https://ve.media.tumblr.com/tumblr_os31dkexhK1wsfqep.mp4
           https://media.tumblr.com/afed9f5b3c33c39dc8c967e262955de2/tumblr_inline_os31dclyCR1v11u29_1280.png
         ]
 
@@ -178,5 +178,13 @@ module Sources
         assert_match("test answer", @site.artist_commentary_desc)
       end
     end
+
+    context "A deleted tumblr post" do
+      should "work" do
+        site = Sources::Strategies.find("http://shimetsukage.tumblr.com/post/176805588268/20180809-ssb-coolboy")
+
+        assert_nothing_raised { site.to_h }
+      end
+    end
   end
 end