post queries: raise error on invalid searches.

Raise an error if the search is invalid for one of the following reasons:

* It contains multiple conflicting order: metatags (e.g. `order:score order:favcount` or `ordfav:a ordfav:b`).
* It contains a metatag that can't be used more than once: (e.g. `limit:5 limit:10`, `random:5 random:10`).
* It contains a metatag that can't be negated (e.g. `-order:score`, `-limit:20`, or `-random:20`).
* It contains a metatag that can't be used in an OR clause (e.g. ` touhou or order:score`, `touhou or limit:20`, `touhou or random:20`).

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(PostQueryBuilder::TagLimitError) do
+            assert_raises(PostQuery::TagLimitError) do
               @set.posts
             end
           end