Most of the time from Sharepoint we will not get the developer friendly error message.
"An unknown error has occured, please contact your system administrator".
To get know more about the error details, please do the following settings.
1. Open your Sharepoint application web.config file.
2. Find out "CallStack" in the web.config file.
3. Change the CallStack="false" value to CallStack="true".
4. By default CustomError mode will be "on" and this value to "Off".
5. <compilation batch=”false” debug=”false“> and change the “false” value of debug to “true”.
6. Save the web.config file do IISReset.
7. When you refresh the Error page, now, you will get the exact error message from Sharepoint.
8. The above settings are not recommended for Production Environment.