When cherry_picker fails, drop you off in the checkout

[brettcannon]

I just had a cherry-pick fail, but because I was running cherry_picker from its own git repo it took me a minute to realize that I couldn't find the failed branch because I had not cd cpython yet. It might work out better to automatically cd into the checkout upon failure (with an appropriate message that this was done).

[Mariatta]

Sorry about that.. yeah it's definitely something that I need to address. Maybe related to #45 too

[brettcannon]

@Mariatta Nothing to apologize for! Tool worked out great (only issue is GitHub seems to keep thinking I might want to create a PR for the 3.5 branch)!

[ncoghlan]

If we did decide to do this (and I think it's reasonable for the interactive case), it would be helpful if it printed two things before exiting:

• a command to finish just the "commit-and-push-and-open-web-browser" steps for the current target branch
• a command to run the cherry-pick again for any not yet processed branches

[Mariatta]

Hmm.. I couldn't figure out how to actually cd to the cpython directory and let it persist there after cherry_picker.py ended 😅 So instead I'm printing out the message that you need to cd cpython first..

[Mariatta]

Once #99 is merged, this is not needed anymore since the people will be cherry-picking from cpython directory.