Which issue(s) this PR fixes:

nothing

What this PR does / why we need it:

• endpoint api/config checks if the path is valid.
• return status code 404 if needed
• match returned error message with

  fluentd/lib/fluent/plugin/in_monitor_agent.rb

  Lines 55 to 59 in 0f3704f

  	code, header, body = render_json_error(500, {
  	'message '=> 'Internal Server Error',
  	'error' => "#{$!}",
  	'backgrace'=> $!.backtrace,
  	})

Docs Changes:

not needed

Release Note:
not needed