Display a visible notification in UnrealEd if the game scripts assembly failed to build on startup

[enlight]

Right now the game scripts assembly is built (if it's missing) when UnrealEd starts up, and any build errors are displayed in the Output window. However, the build errors get buried in other log messages and it's not easy to tell if the game scripts assembly was actually built or not. If I recall correctly there were some changes to the Output window in 4.5 so maybe it will be easier to spot the build errors, but really a more appropriate solution is either a popup or a toast notification... something that's hard to miss.