Allow debugging errors in server gen js

This change prints a useful message and outputs the file with the issue.

lib/react_on_rails/server_rendering_pool.rb

@@ -51,13 +51,13 @@ def self.server_render_js_with_console_logging(js_code)
  class << self
  private
 
- def trace_messsage(js_code)
+ def trace_messsage(js_code, file_name = "tmp/server-generated.js")
  return unless ENV["TRACE_REACT_ON_RAILS"].present?
  # Set to anything to print generated code.
  puts "Z" * 80
  puts "react_renderer.rb: 92"
- puts "wrote file tmp/server-generated.js"
- File.write("tmp/server-generated.js", js_code)
+ puts "wrote file #{file_name}"
+ File.write(file_name, js_code)
  puts "Z" * 80
  end
 
@@ -77,7 +77,19 @@ def create_js_context
 #{console_polyfill}
  #{bundle_js_code};
  JS
- ExecJS.compile(base_js_code)
+ begin
+ ExecJS.compile(base_js_code)
+ rescue => e
+ file_name = "tmp/base_js_code.js"
+ msg = "ERROR when compiling base_js_code! See #{file_name} to "\
+ "ERROR when compiling base_js_code! See #{file_name} to "\
+ "correlate line numbers of error. Error is\n\n#{e.message}"\
+ "\n\n#{e.backtrace.join("\n")}"
+ puts msg
+ Rails.logger.error(msg)
+ trace_messsage(base_js_code, file_name)
+ raise e
+ end
  else
  if server_js_file.present?
  msg = "You specified server rendering JS file: #{server_js_file}, but it cannot be "\