Add a way to extract the 'errorMessage' directly when a JSON schema validation fails.

[danielaparker]

"I am currently using JSONCONS in C++ for validating JSON against a schema. I was wondering if there's a way to extract the 'errorMessage' directly when a JSON schema validation fails.

In my use case, I have a JSON schema with specific validation rules and I want to provide informative error messages to the end users when their JSON fails to validate against the schema. I noticed that there are 'errorMessage' annotations in the JSON schema draft-07, which allow for custom error messages."

Originally posted by @tvrsdq in #573