Improve facet error messages in animint2dir

[ANAMASGARD]

FIXES #168

What's broken

viz <- list(
  plot = ggplot() + 
    facet_wrap(. ~ Species) +  # formula notation
    geom_point(aes(x, y), data = iris)
)
animint2dir(viz) 

• Current error:
  Error: At least one layer must contain all variables used for facetting

Doesn't tell you what's wrong or how to fix it.

• Workaround that works:
  facet_wrap("Species") # string notation works fine

What I am fixing

1. Detect when formula notation is used in facets
2. Give clear error telling user to use string notation instead
3. Fix happens in animint2dir() - animint2-specific code only

Next commit will add the fix to make tests pass.

[ANAMASGARD]

Before :-

Error: At least one layer must contain all variables used for facetting

After :-

Error: Facet variable not found in data: MissingVar
Available columns: Sepal.Length, Sepal.Width, Petal.Length, Petal.Width, Species
Use string notation like facet_wrap("var") instead of formula notation facet_wrap(. ~ var) ```

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.12%. Comparing base (543d0d9) to head (bf4c37c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #285      +/-   ##
==========================================
+ Coverage   73.09%   73.12%   +0.02%     
==========================================
  Files         164      164              
  Lines        8769     8778       +9     
==========================================
+ Hits         6410     6419       +9     
  Misses       2359     2359              

Flag	Coverage Δ
javascript	80.78% <ø> (ø)
r	69.61% <100.00%> (+0.04%)	⬆️

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[tdhock]

• HEAD=fix-168-facet-error-message much slower for getCommonChunk improved in #238
  

Generated via commit bf4c37c

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	4 minutes and 1 seconds
Installing different package versions	1 minutes and 0 seconds
Running and plotting the test cases	2 minutes and 43 seconds

[ANAMASGARD]

Sir @tdhock The 2 failing tests in JS_coverage are in test-compiler-ghpages.R (GitHub Pages tests), not related to this PR. This is the same race condition issue where parallel CI jobs conflict over the shared test repository.

All facet-related tests pass:

• R_coverage: 2096+ tests passed
• CRAN: passed
• My 3 new facet error message tests pass

The fix is working correctly. Please review and give your feedback Sir .