Assure all API errors are returned in json format

[LefterisJP]

Problem Definition

At the moment all error descriptions in the API are provided in plain text as part of the RPC response. That is sometimes not what a user would expect as can be seen from #1020

Solution

Make sure that all unsucesfull api responses contain an errors key.

Tasklist

• Assure all errors are returned in a json object
• Update documentation of the API to reflect the change
• Add a changelog entry.