Include regular errors in call command response object.

Regular errors, .i.e those logged using MXS_ERROR, are currently not included in the response object returned by call commands. That's mostly ok as call commands typically log errors using a macro that uses both MXS_ERROR and explicitly adds an error to the response. Currently the MXS_ERROR is actually pointless, as they will not end up the log or be returned, but simply be discarded.