fpcalc: return non-zero exit code if fingerprint generation fails

[Former user]

If fpcalc fails to read a file, it still returns the exit code 0. It would be much easier for third-party application developers if the exit status would directly show if the fingerprint was correctly generated or not.

[Lukáš Lalinský]

Fixed on 47db4299f0890fae597daadc0924573ff85c5e25.