handle warnings if stash creation encounters permission issue

[shiftkey]

Overview

Related to #7344

Description

I'm not a huge fan of this current patch, but it's targeted to this specific scenario:

• git stash push runs and creates the stash (including untracked files)
• while doing this, Git encounters a path in the repository it does not have access to
• operation continues, completing the stash but raises a non-zero exit code and adds messages to stderr about the problem path

This patch is supposed to:

• look for the 1 exit code
• inspect stderr to ensure no error: messages exist
• if found, throw an error with the context (emulating previous behaviour)
• otherwise just log whatever was found on stderr and continue

@say25 as you had a concrete reproduction of the permissions denied error, are you able to verify this fix at some stage by building this PR branch locally and trying it with that problem repository?

Release notes

Notes: [Fixed] stashing when untracked path is not accessible will not block checkout

[say25]

@shiftkey Build and tested 5a34e36 locally. I can confirm unblocked the expected workflow.

What happens is:

1. A stash is generated
2. I get an error message saying there are merge conflicts (dismissable, confusing but not horrible)
3. I my changes are carried to the new branch and that a patch file with the same change set is generated.

Log:

I had to redact some of the file names but for the most part you can see what is happening

2019-04-18.desktop.development.log

[shiftkey]

Moving this to ready-to-review because the new error uncovered is related to encountering conflicts after applying stashing, which is a known issue: #7278

2019-04-18T18:01:36.502Z - error: [ui] `git stash pop stash@{0}` exited with an unexpected code: 1.
The stash entry is kept in case you need it again.

error: Your local changes to the following files would be overwritten by merge:
    [files]
Please commit your changes or stash them before you merge.
Aborting

[shiftkey]

@iAmWillShepherd I re-requested a review from you to confirm whether you'd like to see this merged for the beta, or whether you want to investigate alternatives...

[outofambit]

thank you!