Return non-zero exit status if "compare" finds differences

This makes `metastore` more useful in scripts, just to check if it found
anything, in the same way as `diff` and `grep`.

Note that POSIX requires that EXIT_SUCCESS is 0, and EXIT_FAILURE is 1,
so I've defined EXIT_DIFFERENCES as 2 which, while different from the
values `diff` and `grep` use (1 for finding something, 2 for errors),
maintains a certain amount of backwards compatibility with previous
versions of metastore.

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