Connection error is confusing (#105)

Hi,

I ran into this connection error:

````
> trezor-agent  timothy@localhost
2017-04-10 00:22:01,818 ERROR        Connection error: open failed                                                                        [__main__.py:130]
````

I didn't know what was going on, whether there was a problem connecting to localhost or what. I eventually logged into trezor wallet and found that there too, my device was not recognized (probably because I did not unplug it/replug it after updating the HID settings.) Unplugging it and replugging it fixed everything.

trezor_agent/__main__.py

@@ -127,7 +127,7 @@ def handle_connection_error(func):
         try:
             return func(*args, **kwargs)
         except IOError as e:
-            log.error('Connection error: %s', e)
+            log.error('Connection error (try unplugging and replugging your device): %s', e)
             return 1
     return wrapper