Make liquid error rendering optional.

Although the author of the liquid template wants to see these errors, they
probably don't want the visitor to see the liquid errors.  Probably the
best fallback when rendering the page for visitors is to render the empty
string for tags with errors.

test/integration/error_handling_test.rb

@@ -185,4 +185,11 @@ class ErrorHandlingTest < Minitest::Test
       template.render('errors' => ErrorDrop.new)
     end
   end
+
+  def test_disabling_error_rendering
+    template = Liquid::Template.parse('This is an argument error: {{ errors.argument_error }}')
+    template.render_errors = false
+    assert_equal 'This is an argument error: ', template.render('errors' => ErrorDrop.new)
+    assert_equal [ArgumentError], template.errors.map(&:class)
+  end
 end