fix(api): Add missing issueMessage field to ValidatorIssueInput

[nellh]

Adds a missing field required by the schema validator.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 45.66%. Comparing base (85184c8) to head (3a0af73).

Files with missing lines	Patch %	Lines
packages/openneuro-server/src/graphql/schema.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3236      +/-   ##
==========================================
- Coverage   45.66%   45.66%   -0.01%     
==========================================
  Files         595      595              
  Lines       38008    38009       +1     
  Branches     1167     1167              
==========================================
  Hits        17355    17355              
- Misses      20456    20457       +1     
  Partials      197      197              

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