BUGFIX: return correct error codes and add tests

[inteon]

In #33, a bug was introduced in the exit code calculation.
This PR fixes that bug and adds tests that check the error code.

[inteon]

/approve

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[maelvls]

For anyone wondering, here is the context:

This bug was revealed while publishing cmctl to Homebrew: a test assertion was changed from 1 to 129, which led to led to a discussion in person (see the assertion change here).

If you install 2.0.0, you will see that cmctl unexpectedly returns the exit code 129 instead of 1 for most errors. For example:

$ cmctl status certificate --kubeconfig=nonexistant; echo $?
error: stat nonexistant: no such file or directory
129
$ cmctl check api; echo $?
error: error finding the scope of the object: failed to get restmapping: failed to get server groups: Get "https://127.0.0.1:58484/api": dial tcp 127.0.0.1:58484: connect: connection refused
129

Before 2.0.0, cmctl used to return 1 on normal errors, and would exit 128 + signal number when cmctl was terminated mid-flight due to a signal.

I don't think this bug will have affected anyone. The only case where checking whether the exit code is 1 or 130 (due to a SIGINT caused by ctrl+C) or 143 (SIGTERM) would make sense would be when cmctl is run for a long time, which is only true for cmctl check api.

/lgtm