add error_msg to BaseReply failure schema

[guusbertens]

According to the documentation, all BaseReply failure messages must contain payload.error_msg, but there is no mention of this in the JSON schema. As a consequence, the generated Python Pydantic models cannot be used to generate failure messages with an error_msg.

This PR modifies the schema so that an error_msg is present.

This PR does not make error_msg required, even though the docs require it. Doing so would make the generated Pydantic models incompatible with other Python codes that might use these.

[Mythir]

Hey Guus, thanks for the PR! This issue has been on our radar since you brought it up during the meeting we had. We still have to make a final decision on it, but I'll see if I can expedite it a little bit. Maybe we can even go a bit further than your suggested change and unify all the failure messages into a single schema.