[experimental] small api refactor

__n.b.__ not sure if this is something that is worth adding or not, just started doing it as i was reading through `api.go`.

small refactor to clean up all the `http.Error(...)` responses. It expanded into returning a byte response as well.

the methods `handleList()` and `handleLog()` have eventsource handlers, and I'm not sure what would happen if I changed their method signatures to mirror the others and use the `handler()` function. doing so would cause the method return to write an empty byte slice to the connection. i.e.:

```go
// api.go:304
if isStreamAccept(r.Header.Get("Accept")) {                                                        
         eventsource.Handler(func(_ string, enc *eventsource.Encoder, stop <-chan bool) {               
                 a.streamLog(h, container.Logs(), enc, stop)
         }).ServeHTTP(w, r)                                                                                                                                                     
         return []byte(""), http.StatusOK, nil
}
```
i don't believe writing an empty byte slice to the connection will have any affect. i have an additional commit with those methods changed and the tests pass.