userpref: Fix wrong documentation about return values of userpref_read_system_buid()
userpref: Fix wrong documentation about return values of userpref_read_system_buid()

Also, add a debug message that will print the actual error code.

patweb/libimobiledevice,upbit/libimobiledevice,libimobiledevice-win32/libimobiledevice,libimobiledevice/libimobiledevice,Tatsh/libimobiledevice,patweb/libimobiledevice,Tatsh/libimobiledevice,libimobiledevice/libimobiledevice,libimobiledevice-win32/libimobiledevice,patweb/libimobiledevice,upbit/libimobiledevice,kalev/libimobiledevice,libimobiledevice-win32/libimobiledevice,kalev/libimobiledevice,libimobiledevice-win32/libimobiledevice,Tatsh/libimobiledevice,kalev/libimobiledevice,upbit/libimobiledevice,Tatsh/libimobiledevice