discord: fix tag search commands being limited to 2 tags.

test/unit/post_query_builder_test.rb

@@ -1045,7 +1045,7 @@ class PostQueryBuilderTest < ActiveSupport::TestCase
     should "fail for more than 6 tags" do
       post1 = create(:post, rating: "s")
 
-      assert_raise(::Post::SearchError) do
+      assert_raise(PostQueryBuilder::TagLimitError) do
         Post.user_tag_match("a b c rating:s width:10 height:10 user:bob")
       end
     end

test/unit/post_sets/post_test.rb

@@ -82,7 +82,7 @@ module PostSets
           should "fail" do
             @set = PostSets::Post.new("a b c", user: create(:user))
 
-            assert_raises(::Post::SearchError) do
+            assert_raises(PostQueryBuilder::TagLimitError) do
               @set.posts
             end
           end