Make .json() work for EnumError

[dmontagu]

Change Summary

Fix problem with the .json() method for EnumError.

Related issue number

#696

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• HISTORY.rst has been updated
  • if this is the first change since a release, please add a new section
  • include the issue number or this pull request number #<number>
  • include your github username @<whomever>

[codecov]

Codecov Report

Merging #697 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #697   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          15     15           
  Lines        2722   2722           
  Branches      538    538           
=====================================
  Hits         2722   2722

[samuelcolvin]

great thanks.