improve error message on client when host/port is bad

for host name without port, None was used for the port
leading to a confusing error message

- do not call parse_host_port with None as defaultport argument
- improve error message when connection fails

+ fix an error in last line of parse_ipv6_host_and_port
+ fix some issues breaking PEP 8 rules

Change-Id: I437360be96449c164f0080e3c60f1685825d4780
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/31911
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

frappy/lib/asynconn.py

@@ -35,7 +35,7 @@ import time
 import re
 
 from frappy.errors import CommunicationFailedError, ConfigError
-from frappy.lib import closeSocket, parse_host_port
+from frappy.lib import closeSocket, parse_host_port, SECoP_DEFAULT_PORT
 
 try:
     from serial import Serial
@@ -176,11 +176,11 @@ class AsynTcp(AsynConn):
             uri = uri[6:]
         try:
 
-            host, port = parse_host_port(uri, self.default_settings.get('port'))
+            host, port = parse_host_port(uri, self.default_settings.get('port', SECoP_DEFAULT_PORT))
             self.connection = socket.create_connection((host, port), timeout=self.timeout)
         except (ConnectionRefusedError, socket.gaierror, socket.timeout) as e:
             # indicate that retrying might make sense
-            raise CommunicationFailedError(str(e)) from None
+            raise CommunicationFailedError(f'can not connect to {host}:{port}, {e}') from None
 
     def shutdown(self):
         if self.connection: