release planner : separate operations that can not be undone from simpler preparations

[romainmenke]

In the release planner we do a lot of things that can error and stop the whole process.
We also do a lot of things that write to disk or (obviously) push to npm.

At this time they are a bit interleaved and there isn't a good structure to the codebase to make things better.

Ideally we should separate the work in two distinct stages :

• all the preparations and validation
• all the write ops

This should make it possible to have better error handling and clearer error messages.

It should also make it safer and easier to add new features in the future.