Get error output from an double-clicked app
This answer is from 2013 but is mostly still relevant.


  Prior to Mountain Lion, all processes managed by launchd, including
  regular applications, had their stdout and stderr file descriptors
  forwarded to the system log. In Mountain Lion and above, stdout and
  stderr go nowhere for launchd managed applications. Only messages
  explicitly sent to the system log will end up there.
  
  If you're writing an application and would like some output to show up
  in the console, then adopt an API built on syslog(3) or asl(3)
  instead. NSLog is one such API, and it has the advantage of logging to
  stderr too so you can easily see your output no matter how you've
  launched your application. If you'd like that functionality but want
  to use asl or syslog directly then you'll want to look in to the
  ASL_OPT_STDERR option to asl_open, and the LOG_PERROR option to
  openlog respectively.


Basically, when you double-click an app (same as /usr/bin/open /Applications/SomeApp.app) there is no stdout/stderr. The dev is expected to send any relevant output/errors to the available logging APIs such as NSLog.