BURs: allow failed BURs to be reapproved.

Fix it so that you can reapprove a failed BUR to run it again. Before
this would fail because it would end up trying to create the aliases
or implications again, which would fail because they already existed.
Now it ignores when an alias or implication already exists. It will
however finish tagging the posts if they haven't been fully moved.

test/unit/bulk_update_request_test.rb

@@ -539,6 +539,42 @@ class BulkUpdateRequestTest < ActiveSupport::TestCase
       end
     end
 
+    context "when a bulk update request fails" do
+      should "allow it to be approved again" do
+        @post = create(:post, tag_string: "foo aaa")
+        @bur = create(:bulk_update_request, script: "alias foo -> bar")
+
+        TagAlias.any_instance.stubs(:process!).raises(RuntimeError.new("oh no"))
+        @bur.approve!(@admin)
+        assert_raises(RuntimeError) { perform_enqueued_jobs }
+
+        assert_equal("aaa foo", @post.reload.tag_string)
+
+        assert_equal("failed", @bur.reload.status)
+        assert_not_nil(@bur.forum_topic)
+        assert_equal(@admin, @bur.approver)
+
+        @ta = TagAlias.find_by!(antecedent_name: "foo", consequent_name: "bar")
+        assert_equal("active", @ta.status)
+        assert_equal(@admin, @ta.approver)
+        assert_equal(@bur.forum_topic, @ta.forum_topic)
+
+        TagAlias.any_instance.unstub(:process!)
+        @bur.approve!(@admin)
+        perform_enqueued_jobs
+
+        assert_equal("aaa bar", @post.reload.tag_string)
+
+        assert_equal("approved", @bur.reload.status)
+        assert_not_nil(@bur.forum_topic)
+        assert_equal(@admin, @bur.approver)
+
+        assert_equal("active", @ta.reload.status)
+        assert_equal(@admin, @ta.approver)
+        assert_equal(@bur.forum_topic, @ta.forum_topic)
+      end
+    end
+
     should "create a forum topic" do
       bur = create(:bulk_update_request, reason: "zzz", script: "create alias aaa -> bbb")