Fix misnamed variable in rebuild command

[savetheclocktower]

This is a fix for the bug I described here. This is an artifact of an earlier rewrite and updated exception handling; stderr now exists in another scope and is thrown, so we should catch it and treat error like a string.

I also noticed that, when stderr is empty, we just reject with an empty string in forkNpmRebuild. In that case, I figure a generic message with the failure code is better than an empty string, so I changed it. Happy to take that out if it bothers anyone.

I've not made much progress in figuring out how to hit this particular code path on demand, though! If I find a way, I'll update the PR.

[DeeDeeG]

I can confirm the stderr should be error now.

(It used to be stderr at this point in the history: https://github.com/pulsar-edit/ppm/blame/2cb92e55808c6c6f2011324d4b229be3035a552f/src/rebuild.js#L72-L74 but a refactor changed the variable in question's name to error without changing what's logged.)

[DeeDeeG]

Looks good to me!!!

Thank you for this, when I read the description of the problem I was ready to investigate and open a PR, but lo and behold, here is such a PR. Brilliant.

Optional code suggestion below, otherwise, feel free to land this. 👍