Hi Chip,
You see more detail error in the log windows or have to change the web.config to as Mark Jones says, which is located in c:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\CONFIG (sharepoint 2010 change 15 to 14) plus the web.config of your web application, for see detail error in browser.
Note: Do not make change in the web.config of “c:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\CONFIG” in production server.
Marlon