wip: Handle error when cherry-picking

[Mariatta]

Attempt at handling errors while cherry-picking.

demo: https://asciinema.org/a/108298

When an error occurs, present two options to user:

1. Stop cherry-picking, and continue manually
2. Skip the current branch, and try to cherry-pick the next branch

When 1 is selected, provide hints on how to continue

When 2 is selected, abort the cherry-pick and continue with the next branch

[Mariatta]

This PR attempts to address some of the comments in #51 and #45

In case there's an error in cherry-picking, it will enter an interactive mode, where user can decide whether they want to quit, or to continue with backport of the next branch.

Please see the demo of how this works at https://asciinema.org/a/108298
Looking for feedback about this approach.

Thanks :)

[brettcannon]

I've only watched the video, but one tweak I would suggest making is when you say "aborting ..." I would name what Python version you're aborting for (e.g. "aborting for 2.7 ..." or something), otherwise I initially thought the whole process was aborting.

[serhiy-storchaka]

I think the script always should stop cherry-picking and save the current state in local or global file if it can't be derived from the state of git. It is very unlikely that cherry-picking to 3.5 can be successful after failing cherry-picking to 3.6. The script should be able to skip already passed steps when run with the --continue option.

[brettcannon]

So I guess that would require sorting the branches to make sure it goes from newest to oldest to make sure your failures fall as late as possible?

[Mariatta]

I made #59 so it will now cherry-pick from the newest branch.

[Mariatta]

Closing. Please see #65 instead.