Added status code of response in front of each packet.

CoAP sends a lot of empty 404's or 205's without actual "content". 
When debugging, and you get an empty packet, you're never sure if it was an "ok" or "not found". So I added the response code in front of every response packet in bold.

example output:
![cli](https://cloud.githubusercontent.com/assets/873684/2681313/0a13c0c6-c18c-11e3-9f7a-433f8182310b.png)

The status code is only bold, the red is just settings of my terminal. 
As you can see with the `-o -n` option, it does look kind of weird, but even with observe you might want to know the status code of each packet, so I'm not sure what a good solution could be.

Also elaborated on the error message, might not be very clear *what* is wrong with the URL for first time users.