Engine silently loads ExampleLevel when level not found

[jaydenmilne]

Describe the bug
We pass the name of the level on the command line to the engine, ie holodeck.exe UrbanCity .... If the engine can't find that level, it will instead silently load the ExampleLevel

To Reproduce
Run the integration tests on a build without the correct levels

Expected behavior
The engine should exit instead of silently loading a different level.