Fix to_chars errc value for overflow

[mborland]

Partially addresses: #166

[cppalliance-bot]

An automated preview of the documentation is available at https://167.charconv.prtest.cppalliance.org/libs/charconv/doc/html/index.html

[cppalliance-bot]

An automated preview of the documentation is available at https://167.charconv.prtest.cppalliance.org/libs/charconv/doc/html/index.html

[codecov]

Codecov Report

Attention: 21 lines in your changes are missing coverage. Please review.

Comparison is base (0e3d9cf) 92.30% compared to head (1a83f89) 92.30%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #167   +/-   ##
========================================
  Coverage    92.30%   92.30%           
========================================
  Files           64       64           
  Lines         8316     8316           
========================================
  Hits          7676     7676           
  Misses         640      640           

Files	Coverage Δ
test/github_issue_152.cpp	98.78% <100.00%> (ø)
test/github_issue_152_float128.cpp	100.00% <ø> (ø)
test/to_chars.cpp	100.00% <100.00%> (ø)
...lude/boost/charconv/detail/dragonbox/dragonbox.hpp	97.38% <80.00%> (ø)
...lude/boost/charconv/detail/ryu/ryu_generic_128.hpp	83.62% <0.00%> (ø)
...de/boost/charconv/detail/to_chars_integer_impl.hpp	96.89% <25.00%> (ø)
include/boost/charconv/detail/dragonbox/floff.hpp	73.60% <50.00%> (ø)
src/to_chars.cpp	92.59% <16.66%> (ø)
src/to_chars_float_impl.hpp	79.55% <0.00%> (ø)

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