Return non-zero exit code when errors occur

[rluetzner]

This implements #3 .

One of the tests fails, but it looks like the test fails due to a change in isitup.org's API. It is unrelated to this change and fails on master as well.

I was actually unable to build a working CLI tool and would appreciate the help. From Click's documentation it seems to me that my change will do exactly what I want it to, but I was unable to verify that messages are written to stderr and a non-zero exit code is set.