feat: improve error messages for access token errors

[coryan]

When we fail to generate an access token the errors can be very
confusing. See for example this discussion:

https://issues.apache.org/jira/browse/ARROW-17069

This change is an attempt at making these errors less confusing. They
could be made event better if we had a specific webpage describing
client library authentication and troubleshooting, but we do not have
such a page at the moment.

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #9485 (5e0e930) into main (b529b8c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #9485   +/-   ##
=======================================
  Coverage   94.64%   94.64%           
=======================================
  Files        1485     1486    +1     
  Lines      136977   136982    +5     
=======================================
+ Hits       129637   129651   +14     
+ Misses       7340     7331    -9     

Impacted Files	Coverage Δ
google/cloud/internal/auth_header_error.cc	100.00% <100.00%> (ø)
...oud/internal/oauth2_authorized_user_credentials.cc	100.00% <100.00%> (ø)
...loud/internal/oauth2_compute_engine_credentials.cc	99.06% <100.00%> (ø)
...oud/internal/oauth2_service_account_credentials.cc	97.32% <100.00%> (ø)
google/cloud/storage/internal/curl_client.cc	84.40% <100.00%> (ø)
google/cloud/storage/internal/rest_client.cc	98.79% <100.00%> (+0.10%)	⬆️
...loud/storage/oauth2/authorized_user_credentials.cc	100.00% <100.00%> (ø)
...cloud/storage/oauth2/compute_engine_credentials.cc	100.00% <100.00%> (ø)
...loud/storage/oauth2/service_account_credentials.cc	92.25% <100.00%> (ø)
google/cloud/internal/curl_impl.cc	91.64% <0.00%> (-0.23%)	⬇️
... and 6 more

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[coryan]

/gcbrun

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 5e0e9301452f43e45e68a6027868eb20a43935a2

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• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.