replace 4243 by the docker port

And add a user friendly message:

`FATA[0000] listen tcp 127.0.0.1:2375: bind: address already in use: is docker already running in this machine ? try using another port`
Minor rephrasing. How about:

"is **D**ocker already running **on** this machin**e?** **T**ry using **a different** port"