Updates for v2 based on feedback

[stevengill]

Summary

Updates based on comments on #380 & #381

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 #439 into @slack/bolt@next will increase coverage by 0.61%.
The diff coverage is 29.41%.

@@                 Coverage Diff                  @@
##           @slack/bolt@next     #439      +/-   ##
====================================================
+ Coverage              85.1%   85.71%   +0.61%     
====================================================
  Files                     7        7              
  Lines                   537      532       -5     
  Branches                158      156       -2     
====================================================
- Hits                    457      456       -1     
+ Misses                   52       48       -4     
  Partials                 28       28

Impacted Files	Coverage Δ
src/errors.ts	100% <100%> (+6.06%)	⬆️
src/ExpressReceiver.ts	72.44% <20%> (+1.44%)	⬆️

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[seratch]

@stevengill can you cherry-pick this commit for corresponding changes to Japanese docs? https://github.com/seratch/bolt/commit/37bfd07ee9ecbae8d8d528ad7fb08968332342c5