Issue/PR creation fails if body exceeds max length

[mschoettle]

When the body length of a merge request is too long, the creation on GitHub fails.

There are two issues with this:

• the PR is never created
• the migration continues potential leading to an inconsistent numbering of PRs compared to MRs (if this is needed)

I suggest to detect this issue and stop the process giving the user the ability to shorten the body length.

In our case this happened for a dependency update of node which includes the release notes. It was a change over several versions so there were lots of changes. node includes (or used to?) all commits of a release.

[spruce]

Sounds good. I just don't have the time to add it currently. I would be willing to accept a PR for this feature.