Standard error message shown when Magento throws an error.
There is two ways to see more about how to find out what the details of the error is.
Locate the error in the var /log (Option 1).
Change the file to have the error printed to the screen (Option 2).
When Magento throws a error it is recorded in a file. What you can do is look at the error message and it will display a number.
Like 1065941995823. This number has no real meaning but tells you what number to look for in the error log.
The error log location is [Magento root]/var/report/[random-number]
The error can be printed to the screen. This is useful when new errors are created and you are trouble shooting a problem.
If you change the file [Magento root]/errors/local.xml.sample
to /local.xml then the errors are printed to the screen.
to [Magento root]/errors/local.xml