MapLoader needs better debugging where someone tries to load a map as a bp

[metalgearsloth]

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Maps get serialized now so people need to be specific on which is used.

[ElectroJr]

#3181 sorta fixes this in that it adds a better error, but the command will currently still start the process of loading the map as a grid, when it should really just reject the file in the first place.

I guess a better fix would be to specify whether this is a singular grid or a map in the meta-data? And I guess some #if DEBUG code that checks to make sure that grid files really don't contain any map entities (and that map files contain only one map entity).