Standard way to report errors

[eguzki]

Fixes #80

[eguzki]

ready for review @miguelsorianod

[miguelsorianod]

As this changes the output content (now that I think about it, the same happens with json structured output), should we add some info in the future release notes telling that the shown output of some commands has changed? (in case someone is relying on that)

[miguelsorianod]

Looks good to me.

Just some minor comments about how it works and feedback

[eguzki]

As this changes the output content (now that I think about it, the same happens with json structured output), should we add some info in the future release notes telling that the shown output of some commands has changed? (in case someone is relying on that)

The json structured output does not break backward compatibility, as it is optional.

Yes, indeed, when there are errors, this PR changes the output (in stderr). Adding release notes about this is a good idea