Improve error messages for Azure AD error codes

[peat-psuwit]

This PR improve error messages for 2 cases:

• Provides useful diagnostics for Azure AD code 65001 and 7000218 [1], which can be found when the Azure AD application is mis-configured. Error 65001 could even be resolved by just following the link in the error message.
• Provides links, similar to [1], for other unknown codes. It links to Microsoft's error code website, which should helps when debugging the issue.

[1]

• https://login.microsoftonline.com/error?code=65001
• https://login.microsoftonline.com/error?code=7000218

[github-actions]

Everyone contributing to this PR have now signed the CLA. Thanks!

[didrocks]

Hey! Thanks for this enhancements and making aad-auth better. The code itself looks good and will help a lot of users as we saw bug reports around this.

There are two things to consider before merging:

• can you sign the CLA (you have a link in the bot description) and tell us once done?
• this project is heavily tested. Consequently, those changes should test every error cases handling you added. The corresponding test is in https://github.com/ubuntu/aad-auth/blob/main/internal/aad/aad_test.go#L13 (you need to add 2 cases). This will trigger the mock to behave differently and returning different errors based on the user name. You can see some example of this in https://github.com/ubuntu/aad-auth/blob/main/internal/aad/msal_mock.go#L47.

Do not hesitate if this triggers any questions or if you need any help! Thanks again :)

[peat-psuwit]

I've actually signed CLA right after I submitted this PR, but I guess it has to be checked by a human before an entitlement is added. Anyway, should be solved now.

The test cases are added.

[codecov]

Codecov Report

Merging #193 (b1f06b7) into main (b401372) will increase coverage by 0.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #193      +/-   ##
==========================================
+ Coverage   83.00%   83.24%   +0.23%     
==========================================
  Files          36       36              
  Lines        2913     2948      +35     
  Branches      279      274       -5     
==========================================
+ Hits         2418     2454      +36     
+ Misses        377      376       -1     
  Partials      118      118              

Impacted Files	Coverage Δ
internal/aad/aad.go	92.30% <100.00%> (+2.51%)	⬆️
internal/aad/msal_mock.go	93.42% <100.00%> (+2.19%)	⬆️

... and 2 files with indirect coverage changes

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[denisonbarbosa]

LGTM. Thanks for the changes!
A few minor nitpicks to improve readability, but everything else is great.

[denisonbarbosa]

Looks great now! Thanks for addressing the suggestions.

[didrocks]

Excellent work! Thanks for your help here, I’m sure it will unlock a lot of users who wants to set that up.