FormContext error() should return array with error names as keys.

[robertpustulka]

Fixes #13235

[codecov]

Codecov Report

Merging #13250 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master   #13250      +/-   ##
============================================
- Coverage     90.95%   90.94%   -0.01%     
  Complexity    14069    14069              
============================================
  Files           522      522              
  Lines         36177    36177              
============================================
- Hits          32904    32901       -3     
- Misses         3273     3276       +3

Impacted Files	Coverage Δ	Complexity Δ
src/View/Form/FormContext.php	94.11% <100%> (ø)	29 <0> (ø)	⬇️
src/Cache/CacheRegistry.php	95.83% <0%> (-4.17%)	11% <0%> (ø)
src/Cache/Engine/FileEngine.php	89.28% <0%> (-1.03%)	76% <0%> (ø)

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