I upgraded from 5.1 to 5.2 and now my exceptions are not showing, and I'm seeing the 500 error page, as if I had config.consider_all_requests_local set to false... it's set to true in my development.rb however.
Also, as a sidenote, when the exception happens theres literally 10 seconds of log output in the terminal before rails responds. It's so much log output that I can't even scroll to see if the error is being triggered by something else.
To all the people that find this... here is the problem. I had to remove the web-console gem from the gemfile (leftover from previous Rails versions), and it works now.
gem 'web-console', '>= 3.3.0'