Update merge_feature_defaults to error on invalid property keys

[jeddai]

https://mozilla-hub.atlassian.net/browse/EXP-3503

Pull Request checklist

• Breaking changes: This PR follows our breaking change policy
  • This PR follows the breaking change policy:
    • This PR has no breaking API changes, or
    • There are corresponding PRs for our consumer applications that resolve the breaking changes and have been approved
• Quality: This PR builds and tests run cleanly
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• Tests: This PR includes thorough tests or an explanation of why it does not
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Branch builds: add [firefox-android: branch-name] to the PR title.

[jhugman]

Looks good!

[codecov-commenter]

Codecov Report

Patch coverage: 96.72% and project coverage change: +0.27 🎉

Comparison is base (d43fbb3) 84.11% compared to head (831c988) 84.39%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5620      +/-   ##
==========================================
+ Coverage   84.11%   84.39%   +0.27%     
==========================================
  Files         106      106              
  Lines       11042    11092      +50     
==========================================
+ Hits         9288     9361      +73     
+ Misses       1754     1731      -23     

Impacted Files	Coverage Δ
components/support/nimbus-fml/src/error.rs	15.15% <50.00%> (+2.24%)	⬆️
components/support/nimbus-fml/src/parser.rs	95.13% <96.29%> (-0.01%)	⬇️
...port/nimbus-fml/src/intermediate_representation.rs	96.53% <100.00%> (+2.14%)	⬆️

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