Exception errors are stored as number named files in var/report/ except if the directory permissions are so messed up that Magento either cannot write there or tries to store its cache in the system /tmp directory.

Use an sftp client to locate and read the files in the var/report/ directory.

The hack used in Magento 1.x to show the stack trace on screen instead of writing it to a file is a security hole if not reversed on a live site. The best example available in the old Magento forums was a stack trace posted with clearly visible login credentials. Bad if you get a Google Dork (archived stack trace) off it.

For Magento 2, /errors/local.xml is moved to /pub/errors/local.xml

Copy local.xml.sample to local.xml and make modifications to fit your server environment.