Make http_error_code a parameter of the MergeErrorResponse constructor. It defaults to 500, the value previously hard-coded in the call to the super class constructor. This change will allow us to specify other error codes in places where 500 is not useful. Part of mpuls/issue6009

syntax: glob
**/*.pyc
**/*.pyo
*.swp
.tmp*
*.egg-info
dist/*
build/*
lib/*
*.orig
*.log
.settings/*
data/*
storage/*
.project
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*.db
odfcast/odfcast.ini