Fixed bug where a redefined ansi.style_error was not always being used

[kmvanbrunt]

See #819

[codecov]

Codecov Report

Merging #820 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #820   +/-   ##
=======================================
  Coverage   97.26%   97.26%           
=======================================
  Files          14       14           
  Lines        3510     3510           
=======================================
  Hits         3414     3414           
  Misses         96       96

Impacted Files	Coverage Δ
cmd2/argparse_completer.py	100% <100%> (ø)	⬆️
cmd2/argparse_custom.py	94.23% <100%> (ø)	⬆️

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