Improve git error messaging

[zkat]

Right now, pacote is swallowing all stderr output from the git subprocesses.

We should really pick that up and either included in the error message, or log it to stderr.

[scotttrinh]

@zkat I'd be happy to jump in here. Not sure how Bluebird deals with the presence of all three callback arguments, but I'm sure we can either work around that, or refactor https://github.com/zkat/pacote/blob/latest/lib/util/git.js#L6-L8 to build its promise manually.

Jumping off of your two suggestions:

1. We could say only if there was an error, include the output of stderr in the error object.
2. We could always write non-empty stderr value to process.stderr.

Are there other places in the npm codebase that we can use to decide between these two options (or some other options?) that you know of?

[zkat]

@scotttrinh I like the idea of passing through stderr when there's a nonzero exit code. I'd gladly take a PR and I think you'll come across a lot of concrete answers while you try it out yourself. Try linking your pacote fork into a fork of npm to see what it feels like from the user's perspective, too