Add login rejection detail, if present

[jpdillingham]

Reviewing the Nicotine+ docs, I realized that there's an extra string "detail" if login is rejected because of a bad username. This PR adds support for reading it, and appends it to the rejection reason if present. It also clarifies the Message property, which gives a MOTD message when login is successful and is not strictly used to communicate a rejection reason as previously stated.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.19%. Comparing base (8e260ea) to head (847f5ff).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #968      +/-   ##
==========================================
+ Coverage   99.16%   99.19%   +0.03%     
==========================================
  Files         189      189              
  Lines        6704     6707       +3     
  Branches     1024     1026       +2     
==========================================
+ Hits         6648     6653       +5     
+ Misses         42       41       -1     
+ Partials       14       13       -1     

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