swift-recon --md5sum should exit with an error code on error

[Pete Zaitcev]

<rcernin> zaitcev: actually it would be nice if swift-recon --md5sum returns exit code 1 if there is error reported.
<rcernin> zaitcev: but because it always return 0, we need to check for the [^0] error
<rcernin> zaitcev: if it would return the right exit code, i d just run it without any control structure.

See also: https://review.gerrithub.io/375225

The code does some strange things:

- The wrapper in bin/swift-recon passes the return code to sys.exit(), perfect. BUT the catcher of ^C does not pass it, AND the SwiftRecon.main() does not actually return anything useful, EVER.

- Including strangely enough when it trows a usage help! It invokes sys.exit() explicitly even, which is just rude.

P.S. Swift git tag is 2.14.0 for this