Fix MoveMapGen exit codes.

[mserajnik]

🍰 Pullrequest

Currently, MoveMapGen exits with 1 once it has completed successfully, which would indicate an error.
It also exits with -1, -2 and -3 in various error scenarios but there doesn't seem to be a good reason to differentiate using different codes to begin with and even less reason to use negative values.

This changes the exit codes to use EXIT_SUCCESS and EXIT_FAILURE from the cstdlib header (which should be preferable over using 0 and 1 directly because it makes the code more portable).

Proof

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How2Test

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Todo / Checklist

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