changed: @error instead of @warn in MHE arrival covariance fallback + update CI

[franckgaga]

Following the "convention" throughout the package, a non fatal @error log make more sense than @warn.
It's similar to @error emitted by an unfeasible MPC problem: something is wrong but there is a fallback. That is, take the solution from the last time step.

Also updated the CI.yml with new feature like improved speed with cache v2 and replacing Julia nightly by pre testing.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.89%. Comparing base (cc2a92f) to head (701c5cc).
Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #154   +/-   ##
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  Coverage   98.89%   98.89%           
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  Files          24       24           
  Lines        3800     3800           
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  Hits         3758     3758           
  Misses         42       42           

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[franckgaga]

FYI @baggepinnen

Might be worth it to update the test.yml file in ControlSystem.jl. See https://discourse.julialang.org/t/with-julia-actions-setup-julia-you-can-now-specify-lts-and-pre-named-versions/

The interesting parts would be to update to julia-actions/setup-julia@v2, actions/checkout@v4 and julia-actions/cache@v2. The v2 of cache speeds up the testing but requires this addition in the yml file:

    runs-on: ${{ matrix.os }} # new part just after this line:
    permissions: # needed for julia-actions/cache delete old caches that it has created
      actions: write
      contents: read

With v2 of setup-julia you can execute the tests on:

      matrix:
        version:
          - 'lts' # long-term support release
          - '1'   # latest stable 1.x release
          - 'pre' # latest stable prerelease
          # - 'nightly' # commented since noisy + 'pre' allows testing upcoming versions

IMO it covers large enough to see major issues for Julia version incompatibilities.