gdrive: fix multi-remote workflow, cont. cleanup

[shcheklein]

Address more checkboxes from the #2865.

Specifically:

• We should store one credentials file per remote. Solution: behave optimistically, do not require a config option or complicate default naming (difficult names that depend on remote name may lead to discrepancies in simple scenarios like renaming or changing settings of a single GDrive remote). Better detect discrepancy, cases of access failure due to wrong credentials being used.
• Fix how path for the gdrive_user_credentials_file is handled - it should be resolved against config location if it is relative.
• Cleanup and simplify code in GDrive auth and other error handlers
• Minor cleanup and typos fixed in a few other cases
• Add tests

Docs PR: iterative/dvc.org#1192

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• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here. If the CLI API is changed, I have updated tab completion scripts.

• ❌ I will check DeepSource, CodeClimate, and other sanity checks below. (We consider them recommendatory and don't expect everything to be addressed. Please fix things that actually improve code or fix bugs.)

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

[codecov]

Codecov Report

Merging #3686 into master will decrease coverage by 0.11%.
The diff coverage is 71.15%.

@@            Coverage Diff             @@
##           master    #3686      +/-   ##
==========================================
- Coverage   90.77%   90.65%   -0.12%     
==========================================
  Files         158      158              
  Lines       10001    10018      +17     
==========================================
+ Hits         9078     9082       +4     
- Misses        923      936      +13     

Impacted Files	Coverage Δ
dvc/remote/gdrive.py	80.07% <65.90%> (-3.40%)	⬇️
dvc/config.py	97.71% <100.00%> (ø)
dvc/data_cloud.py	97.05% <100.00%> (ø)
dvc/updater.py	54.65% <0.00%> (-1.17%)	⬇️

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[jorgeorpinel]

Questions and suggestions on cmd output.

[jorgeorpinel]

A few more details for your consideration ☝️