GitLab

- always return command errors in Next, neven in Send
- also check trailer when receiving EOF
- update tests to reflect changes

- remove HandleError() because we now have CloseWithError
- remove Header interface
- remove all Head() methods
- fix locking in chan.go
- use EmitChan in ResponseEmitters
- introduce cmds.ErrClosedEmitter
- fix tests (some still expected errors to be emitted/marshaled)

Some commands return nil on success.

Instead, handle it at the boundaries (in ServeHTTP and the CLI's Run function).
This ensures that we actually *wait* to finish writing everything before
canceling the context.

- made Response.Request() return Request not *Request
- added ResponseEmitter.Tee()
- added a wrapper that wraps a Response and provides an oldcmds.Response