Improve error messages

If I try to create an index on a keyspace that does not exist, a json body is returned with the following error property (string) :

rest_create_index: error creating index: c47fdb, err: manager_api: CreateIndex, Prepare failed, err: bleve: Prepare, err: readObjectStart: expect { or n, but found R, error found in #1 byte of ...|Requested r|..., bigger context ...|Requested resource not found.|...

There is room for improvement 😂.

I think having an object with a reason property (Keyspace not found) and a details property is a good start.