Exit on failing command

[daneov]

Following on #7 this PR introduces/fixes the following behaviour:
In the case of a command failure Komondor will:

• Print the error message and exits with the error code of the failed command
• In case of any other type of error Komondor will exit with status 1 and print the raw description of the error

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Aside from that, I introduced the following small changes:

• Make the 'happy path' more explicit by using a guard to check if arguments are passed or not
• The resulting value from the try shellOut(to: command) is printed - this is only after the command has been executed.
  • ⚠️ I tried passing the correct FileHandler to shellOut in an attempt to print the output in real time, rather than after completing the action, but there are some bugs in shellOut that prevent us from doing this.

[orta]

My preference is to not output by default - but I'm open to giving this a shot 👍