First you have to open the web.confing file of the application C:\inetpub\wwwroot\wss\VirtualDirectories\<port>.
- Debug=”true” instead of the default of Debug=”false”
- CallStack=”true” instead of the default of CallStack=”false”
- CustomErrors=”Off” instead of the default of CustomErrors=”On”
Secondly, there is a second web.config file located at the
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\TEMPLATE\LAYOUTS\web.config
That second web.config file contains the customErrors section that needs to be set to off.
Central Administration has a second web.config file too if you need to see error descriptions there:
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\TEMPLATE\ADMIN\web.config
From customErrors section set the value to off.