Handle missing reference parsers gracefully

When a redacting references and the particular reference is no longer
valid, we should just treat it as an inaccessible reference instead of
raising an exception.

This can happen when are redacting stale HTML that has some references
that we already removed.

lib/banzai/reference_parser.rb

@@ -2,6 +2,8 @@
 
 module Banzai
   module ReferenceParser
+    InvalidReferenceType = Class.new(StandardError)
+
     # Returns the reference parser class for the given type
     #
     # Example:
@@ -11,6 +13,8 @@ module Banzai
     # This would return the `Banzai::ReferenceParser::IssueParser` class.
     def self.[](name)
       const_get("#{name.to_s.camelize}Parser", false)
+    rescue NameError
+      raise InvalidReferenceType
     end
   end
 end

lib/banzai/reference_redactor.rb

@@ -111,6 +111,7 @@ module Banzai
         parser = Banzai::ReferenceParser[type].new(context)
 
         visible.merge(parser.nodes_visible_to_user(user, nodes))
+      rescue Banzai::ReferenceParser::InvalidReferenceType
       end
 
       visible

spec/lib/banzai/reference_redactor_spec.rb

@@ -182,5 +182,12 @@ RSpec.describe Banzai::ReferenceRedactor do
 
       expect(redactor.nodes_visible_to_user([node])).to eq(Set.new([node]))
     end
+
+    it 'handles invalid references gracefully' do
+      doc = Nokogiri::HTML.fragment('<a data-reference-type="some_invalid_type"></a>')
+      node = doc.children[0]
+
+      expect(redactor.nodes_visible_to_user([node])).to be_empty
+    end
   end
 end