Don't I_Error if non base files fail to load with -file

Could the game still show some sort of prompt if a specified file isn't loaded? It doesn't have to be an I_Error, but something to make it obvious instead of the player mysteriously not having a file because they misspelled the name or something.

changed milestone to %2.2.1

This was done for map editors that launch the game with the same file twice. I'm not sure exactly how that works since I've never used the feature, but it seems popping a text box would be undesirable.

merged

mentioned in commit c46d9a1b

...I just realized you meant if the file wasn't found. Yeah I'll do something for that.

mentioned in merge request !753 (merged)

mentioned in commit 8054f341