Write errors to stderr

[bluesign]

Closes #669

Description

This should be discussed as it is a breaking change and can break someone's workflow.

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For contributor use:

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[devbugging]

This is good, thank you. Just wondering how to make it so it doesn't break for someone that was piping outputs etc. I feel if we keep both for some time won't help much as anyhow we can't detect usage of each stream. Maybe just an appropriate note in release notes could be enough. Any thoughts/ideas?

[bluesign]

@sideninja that is my concern too actually, maybe we announce and then merge in few releases ahead?