Harden behavior on failed changeset read #175

Currently we always assume in ‘version’ that changesets are formatted correctly. This is because in v1, we strongly discouraged editing the json file.

In v2 we’ve started to say “well you could write these manually” but I haven’t battle-tested the failure oaths here. We should do that, and make sure the way it fails is explanatory (and that it fails)

The main cases are:

• no frontmatter at all
• frontmatter is malformed (cannot he turned to json, or possibly has incorrect data structures)
• wants to bump packages that don’t exist (the two cases here are a package has been removed, or a typo in a package name - this one keeps me up some at night)