unified error situation handling
unified error situation handling

With regard to posting to the docker host there are two failure scenarios:
 * The request fails low level (i.e. no connection).
 * The request fails on application level (404).

In both cases the post handlers should return an error. In the second case the error message is contained in the
response body. Read that and return it in the error message.

dynport/dgtk,dynport/dgtk,dynport/dgtk,dynport/dgtk