In some of the NSIS Installer documentation, it says that if some operation fails, the error flag will be set. Is the error flag just a boolean value or is a specific code that indicates what error occurred? If it is a code, how do I access it to log it out?
It is just a boolean flag, use ClearErrors, IfErrors and SetErrors to clear/detect/set it. (This flag is set by the internal instructions, most plugins use other methods but it is also possible for a plugin to set/clear it)