wireprotov2: change command response protocol to include a leading map The error handling mechanism for the new wire protocol isn't very well-defined. This commit takes us a step in the right direction by introducing a leading CBOR map for command responses. This map will contain an overall result of the command. Currently, the map indicates whether the command was overall successful or if an error occurred. And if an error occurred, that error is present in the map. There is still a dedicated error frame. My intent is to use that for protocol-level errors and for errors that are encountered after the initial response frame has been sent. This will be clarified in a later commit. Differential Revision: https://phab.mercurial-scm.org/D3385