Izpack 1109

This PR fixes the NPE aspect of IZPACK-1109. It turns out that the problem was present when using the 'complex' conditions as well. With these changes, a warning is printed when any operand of a condition doesn't exist, and the condition will evaluate to false if this is the case. There are a few additional things that I discovered and would like to discuss.

a) The conditions for a panel are checked many times when switching, up to 4 times in a row in some cases. 
b) The logging of the missing conditions can get quite verbose if using complex nested conditions.
c) The error messages that I have added (and perhaps throughout the class?) should probably be localized.
d) The changes to configureVisibility() could have repercussions I am not aware of due to panel actions.

More details about the new logging can be found in the JIRA. Also, sorry for the incorrect JIRA reference in the commit message.