Fix unhandled rejected promises and fix tests

[DonJayamanne]

Fixes #1918
Fixes #1919

This pull request:

• Has a title summarizes what is changing
• Includes a news entry file (remember to thank yourself!)
• Has unit tests & code coverage is not adversely affected (within reason)
• Works on all actively maintained versions of Python (e.g. Python 2.7 & the latest Python 3 release)
• Works on Windows 10, macOS, and Linux (e.g. considered file system case-sensitivity)

[codecov]

Codecov Report

Merging #1921 into master will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1921      +/-   ##
==========================================
+ Coverage   74.36%   74.44%   +0.07%     
==========================================
  Files         282      282              
  Lines       13279    13279              
  Branches     2387     2387              
==========================================
+ Hits         9875     9885      +10     
+ Misses       3270     3261       -9     
+ Partials      134      133       -1

Impacted Files	Coverage Δ
src/client/debugger/mainV2.ts	80.32% <0%> (+0.4%)	⬆️
...rc/client/debugger/PythonProcessCallbackHandler.ts	54.27% <0%> (+0.65%)	⬆️
src/client/debugger/PythonProcess.ts	51.25% <0%> (+1.66%)	⬆️
src/client/common/installer/channelManager.ts	96.36% <0%> (+5.45%)	⬆️

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[d3r3kk]

Seems fine!