exit codes for command line tool

[hannob]

We should have some strategy for exit codes for the commandline tool, currently badkeys doesn't set any.

It is a bit nontrivial, as we can scan multiple keys, and there can both be non-cryptographic warnings and vulnerabiltiy findings.

I am thinking about the following:

• Return 0 if only "good" keys without vulnerabilities have been scanned.
• Return 1 if any vulnerability has been found
• Return 2 if any other problem happened
• Return 3 if 1 and 2 would apply