Prevent removal of ErrorSpecNode from execution hierarchy

[leonard84]

As the ErrorSpecNode does not have children it would get removed
when trying to select specific tests methods
For example, gradle will report that no test were found,
and not report the actual error.

[codecov]

Codecov Report

Merging #1358 (d5c3745) into master (9da9608) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #1358   +/-   ##
=========================================
  Coverage     79.22%   79.22%           
- Complexity     3943     3946    +3     
=========================================
  Files           408      408           
  Lines         12262    12263    +1     
  Branches       1617     1616    -1     
=========================================
+ Hits           9714     9715    +1     
  Misses         1967     1967           
  Partials        581      581           

Impacted Files	Coverage Δ
...java/org/spockframework/runtime/ErrorSpecNode.java	83.33% <100.00%> (+3.33%)	⬆️

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