Set App.ErrWriter in App.Setup()

[Nokel81]

What type of PR is this?

(REQUIRED)

• bug
• cleanup
• documentation
• feature

What this PR does / why we need it:

(REQUIRED)

This change fixes App.ErrWriter not being initialized in App.Setup() like App.Writer. It also removes the App.errWriter() private method since App.ErrWriter is now set directly.

Which issue(s) this PR fixes:

(REQUIRED)

Fixes #1096

Release Notes

(REQUIRED)

- `App.ErrWriter` now defaults to `os.Stderr` if not set.

[lynncyrin]

⭐⭐⭐ (must change) It's probably gonna be annoying to write, but I would like to see a test for this behavior!

[codecov]

Codecov Report

Merging #1100 into master will increase coverage by 0.19%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1100      +/-   ##
==========================================
+ Coverage   73.07%   73.26%   +0.19%     
==========================================
  Files          34       34              
  Lines        2488     2484       -4     
==========================================
+ Hits         1818     1820       +2     
+ Misses        564      559       -5     
+ Partials      106      105       -1

Impacted Files	Coverage Δ
app.go	83.4% <66.66%> (+2.17%)	⬆️

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[lynncyrin]

✨thanks! ✨