Multi-instance support

Only works in the GUI for now, doesn't break  the command-line client but the new options are not implemented.

Related to #29 
Let's change it to ErrorFormat, I changed it because the GUI sometimes tries to uninstall mods which have been uninstalled automatically by the previous call to Uninstall() (because they depended on a mod that was uninstalled). I will see how to fix this GUI-side.