FEAT - Error ID for JHOVE GUI

[carlwilson]

• added node for Error IDs where present in GUI window; and
• added serialisation ID for RepTreeRoot class.

Closes #519

[codecov]

Codecov Report

Merging #546 into integration will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff              @@
##             integration    #546   +/-   ##
=============================================
  Coverage           45.8%   45.8%           
  Complexity          1053    1053           
=============================================
  Files                 57      57           
  Lines               9150    9150           
  Branches            1686    1686           
=============================================
  Hits                4191    4191           
  Misses              4415    4415           
  Partials             544     544

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