Create a BlockBody class to decouple block body parsing from tags.

test/integration/document_test.rb

@@ -0,0 +1,19 @@
+require 'test_helper'
+
+class DocumentTest < Minitest::Test
+  include Liquid
+
+  def test_unexpected_outer_tag
+    exc = assert_raises(SyntaxError) do
+      Template.parse("{% else %}")
+    end
+    assert_equal exc.message, "Liquid syntax error: Unexpected outer 'else' tag"
+  end
+
+  def test_unknown_tag
+    exc = assert_raises(SyntaxError) do
+      Template.parse("{% foo %}")
+    end
+    assert_equal exc.message, "Liquid syntax error: Unknown tag 'foo'"
+  end
+end