fix: update error messages to specify "Invalid GitGuardian API key"

[mathieubellon]

Context

Our error message ("Error: Invalid API key.") is too generic. In some cases, it confuses developers, who may suspect that their Git credentials or token are the source of the problem, leading to a costly waste of time.

What has been done

We changed the error strings to be more specific.

PR check list

• As much as possible, the changes include tests (unit and/or functional)
• If the changes affect the end user (new feature, behavior change, bug fix) then the PR has a changelog entry (see doc/dev/getting-started.md). If the changes do not affect the end user, then the skip-changelog label has been added to the PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.99%. Comparing base (c3d4753) to head (3e399fc).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1120   +/-   ##
=======================================
  Coverage   91.99%   91.99%           
=======================================
  Files         144      144           
  Lines        6162     6162           
=======================================
  Hits         5669     5669           
  Misses        493      493           

Flag	Coverage Δ
unittests	91.99% <100.00%> (ø)

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[sevbch]

To fix the CI, run this:

• pre-commit install
• pre-commit run --all-files