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

@@ -2,12 +2,17 @@
 # :markup: markdown
 
 begin
-  require "ruby_parser"
+  require "sexp"
 rescue LoadError
-  warn(%q{Error: Unable to load ruby_parser. Add `gem "ruby_parser"` to your Gemfile.})
+  warn(%q{Error: Unable to load sexp. Add `gem "sexp"` to your Gemfile.})
   exit(1)
 end
 
+class RubyParser # :nodoc:
+  class SyntaxError < RuntimeError # :nodoc:
+  end
+end
+
 module Prism
   module Translation
     # This module is the entry-point for converting a prism syntax tree into the