At the moment, if you place a hold via the Holds API and there are too many holds for the record, you get the following response:

{"error":"Hold cannot be placed. Reason: tooManyHoldsForThisRecord"}

That's a message that is meant for a human developer. It's not showable to a human audience, and it's not easily parsed by a machine. 

It would be good to have a human readable message for troubleshooting, but the core error code should just be "tooManyHoldsForThisRecord", as a machine can take that string and work with it as a code.