Selectively reduce multithreaded parsing @error

[Liozou]

Reduce the @error introduced in #1073 to a @warning whenever previous warnings may explain why multithreaded parsing failed (typically, unequal number of columns per row). In those cases, it will print:

┌ Warning: Multithreaded parsing failed and fell back to single-threaded parsing, check previous warnings for possible reasons.

If there is no possible explanation, this still shows a red @error saying:

┌ Error: Multithreaded parsing failed and fell back to single-threaded parsing. This can happen if the input contains multi-line fields; otherwise, please report this issue.

In both cases, a debug statement is printed as soon as multithread parsing failed (if the Context debug kwarg is set), whereas the @warning or @error only appears at the end of the fallback single-threaded parsing.

I think this compromise allows us to still get error reports like #1095, which are crucial to detect and allow use to fix underlying issues where multithreaded parsing should not fail (like #1098 in this instance), while avoiding to stress the user with an @error when there is nothing to report.

Close #1095.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 🎉

Comparison is base (07fb6c2) 90.40% compared to head (5093e6b) 90.42%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1099      +/-   ##
==========================================
+ Coverage   90.40%   90.42%   +0.01%     
==========================================
  Files           9        9              
  Lines        2294     2298       +4     
==========================================
+ Hits         2074     2078       +4     
  Misses        220      220              

Impacted Files	Coverage Δ
src/context.jl	88.99% <100.00%> (ø)
src/file.jl	94.46% <100.00%> (+0.03%)	⬆️

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[quinnj]

Thanks!