Return None on exception in server_version

dblogin does not except to receive an exception in test_server_version.
Raising an exception makes the dialog freeze.
The raise was introduced probably by mistake by changeset 0eb1d501ca9d.

issue5789
review31451003

tryton/rpc.py

@@ -58,7 +58,7 @@ def server_version(host, port):
         logging.getLogger(__name__).debug(repr(result))
         return result
     except (Fault, socket.error):
-        raise
+        return None
 
 
 def login(host, port, database, username, parameters, language=None):