Support the error interface in docker v1.10

[mrkschan]

The connection error is changed to something like..

An error occurred trying to connect: Get http://127.0.0.1:2375/v1.22/containers/json: dial tcp 127.0.0.1:2375: connectex: No connection could be made because the target machine actively refused it.

docker version
Client:
 Version:      1.10.2
 API version:  1.22
 Go version:   go1.5.3
 Git commit:   c3959b1
 Built:        Mon Feb 22 22:37:33 2016
 OS/Arch:      windows/amd64

[tiangolo]

Thanks @mrkschan !

And thanks for checking for both error strings (so that we don't break backwards compatibility).