fix: [users] accepting registration requests can throw a badly mapped exception
fix: [users] accepting registration requests can throw a badly mapped exception

- changed to 400

MISP/MISP,SteveClement/MISP,SteveClement/MISP,MISP/MISP,SteveClement/MISP,MISP/MISP,SteveClement/MISP,MISP/MISP,SteveClement/MISP,MISP/MISP