Define RubyParser::SyntaxError directly and drop require for ruby_parser.

Had to add a require of sexp since that came in indirectly via ruby_parser.

Author: zenspider

lib/prism/translation/ruby_parser.rb

@@ -1,11 +1,10 @@
 # frozen_string_literal: true
 # :markup: markdown
 
-begin
-  require "ruby_parser"
-rescue LoadError
-  warn(%q{Error: Unable to load ruby_parser. Add `gem "ruby_parser"` to your Gemfile.})
-  exit(1)
+require "sexp"
+
+class RubyParser
+  SyntaxError = Class.new RuntimeError
 end
 
 module Prism