I had the same error when first installing CyanogenMod 10.1 on my phone. First, make sure you have the correct version of CyanogenMod for your device (make sure it is for the GT-I9100, and not the GT-I9100T or other variants) and move the files to your computer.
Extract the .zip file.
Once unzipped, browse to the META-INF/com/google/android directory.
You will find two files called “update-binary” and “updater-script”.
Rename “updater-script” to “updater-script.txt” and open with a text file editor.
Delete the line starting with “assert” to first semi-colon, including the semi-colon. Usually this is the first line or one of the first lines at the top of the text file.
Save the file and rename it back to "updater-script".
Re-zip the new, edited files and copy to your phone (make sure you copy the edited file, not the original by accident).
Install the new, edited .zip in recovery and all should be fine
Also, try using different versions of CWM if this doesn't work first time. You can use the "ROM Manager" app to install different versions of CWM, try older or newer versions if installing CyanogenMod does not work with the edited .zip file.
Sent from my GT-I9100 using xda app-developers app