Make App.handleError() private (v2)

[aoberoi]

Summary

The App.handleError() method used to be used by the ExpressReceiver before #439. Receivers no longer need this method because they are strictly downstream of the event handling in an app. Therefore it makes sense to turn this into a private method.

Depends on #440 (integration tests will fail without it)

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #446 into @slack/bolt@next will not change coverage.
The diff coverage is n/a.

@@                Coverage Diff                @@
##           @slack/bolt@next     #446   +/-   ##
=================================================
  Coverage             85.71%   85.71%           
=================================================
  Files                     7        7           
  Lines                   532      532           
  Branches                156      156           
=================================================
  Hits                    456      456           
  Misses                   48       48           
  Partials                 28       28

Impacted Files	Coverage Δ
src/App.ts	88.4% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ffb48ee...d4c9fae. Read the comment docs.