fix the location for yarn-error.log and add a test case

[clanghout]

Summary
In order to change the folder where yarn-error.log is placed, we changed the path from config.cwd to config.cacheFolder. In the process of testing we also discovered a bug with specifying a cache folder from the command line.
The test executes a command that will error a yarn-error.log and verifies that this log is created in the specified cache folder.
Fixes #2719

Test plan
Run yarn test

or by hand: run the command yarn tag rm non-existing-pkg non-existing-tag and press enter, then verify that the reporter put the error log in the cache folder.

[TimvdLippe]

It seems that CircleCI is timing out on __tests__/commands/install/integration.js (357.669s) and the same for AppVeyor: __tests__\commands\install\integration.js (275.532s)

On Travis all succeeded, but the 1 job on OS X is failing. That might be a mac related issue.

[TimvdLippe]

On all CI's, the integration test is timing out. However, our own test case is not the culprit any longer. The last commit fixes the path issue on OS X.

[bestander]

Yarn puts the error log into CWD the same way npm does.
Why would we want it in cache folder?

[bestander]

This is done on purpose afaik.
@arcanis we probably need to add a comment here.

[clanghout]

This probably does need a change to either _cacheRootFolder or cacheFolder since cacheRootFolder does not exist in the configOptions export type. The reason for this change was the cache folder location not changing when setting it with the --cache-folder command (see #2881).

[arcanis]

Yep the specific line where this is used is here: https://github.com/yarnpkg/yarn/blob/master/src/config.js#L241

In order to prevent BC break we decided to keep using cacheFolder in the public interface instead of cacheRootFolder, but it seems I forgot one. Good catch @clanghout!

[clanghout]

Since the release of npm v4.2.0 the error logs are placed in the cache folder instead of the CWD:

This debug log would get cleared out as soon as you ran npm again. This was a bit annoying because 1) you would get a random file in your git status that you might accidentally commit, and 2) if you hit a hard-to-reproduce bug and instinctually tried again, you would no longer have access to the repro npm-debug.log.

The cache will now hold a (configurable) number of npm-debug.log files, which you can access in the future.

source: npm release v4.2.0.
What differs however (and I have not implemented) is saving multiple log files inside the cache folder.

[TimvdLippe]

Using process.exec does not generate any coverage. We might be able to directly require cli/index.js and then pass the arguments.

[bestander]

Still it seems weird to have the error in the cache folder.
I understand the risk is people may commit it accidentally.

[bestander]

I guess doing same as npm with <cache>/_logs and adding timestamp to the filename could be an option.
And also Yarn should log the error file location if it does not.

[clanghout]

Different behavior of the cache folder is implemented in #3367 and the found bug is merged in #3033