Use BOOST_OVERRIDE

[EugeneZelenko]

Use BOOST_OVERRIDE to fix GCC -Wsuggest-override and Clang-tidy modernize-use-override warnings.

Also fix Clang-tidy readability-container-size-empty and readability-redundant-string-init warnings.

[codecov]

Codecov Report

Merging #28 into develop will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff            @@
##           develop      #28   +/-   ##
========================================
  Coverage    63.23%   63.23%           
========================================
  Files           30       30           
  Lines          718      718           
  Branches       279      286    +7     
========================================
  Hits           454      454           
  Misses         108      108           
  Partials       156      156           

Impacted Files	Coverage Δ
include/boost/contract/core/exception.hpp	92.85% <ø> (ø)
...e/boost/contract/detail/inlined/core/exception.hpp	35.36% <0.00%> (ø)

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