remote_settings: Change the ConfigError message from ... to a helpful message.

[linabutler]

Oops! I caught this as soon as #6191 made it through the merge queue 🤦🏼‍♀️

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
  • Note:
    • For changes that need extra cross-platform testing, consider adding [ci full] to the PR title.
    • If this pull request includes a breaking change, consider cutting a new release after merging.
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry in CHANGELOG.md or an explanation of why it does not need one
  • Any breaking changes to Swift or Kotlin binding APIs are noted explicitly
• Dependencies: This PR follows our dependency management guidelines
  • Any new dependencies are accompanied by a summary of the due dilligence applied in selecting them.

Branch builds: add [firefox-android: branch-name] to the PR title.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 83.85%. Comparing base (0152f26) to head (f7109cf).
⚠️ Report is 1125 commits behind head on main.

Files with missing lines	Patch %	Lines
components/remote_settings/src/error.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6193   +/-   ##
=======================================
  Coverage   83.85%   83.85%           
=======================================
  Files         117      117           
  Lines       15671    15671           
=======================================
  Hits        13141    13141           
  Misses       2530     2530           

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