Exit rather than return upon version failure (#14)

A plain `zig build` was exiting as expected after the version check
failure did an early 'return'.

But `zig build 01` (specifying a step) would print the fail message
and then continue...but fail to find the step (which was not added).

Calling `exit()` solves this. Calling it with an error status code
causes vomiting in the build system (since, you know, there was an
error). So returning with `exit(0)` is the way to go here.

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