Gracefully handle log following output

If the Zookeeper node goes away (e.g. VM is deleted), return a
reasonable error message instead of a stack trace.

client-common/vm.py

@@ -529,6 +529,7 @@ def follow_console_log(zk_conn, domain, lines=10):
     # Print the initial data and begin following
     print(loglines, end='')
 
+    try:
         while True:
             # Grab the next line set
             new_console_log = zkhandler.readdata(zk_conn, '/domains/{}/consolelog'.format(dom_uuid))
@@ -553,6 +554,10 @@ def follow_console_log(zk_conn, domain, lines=10):
                 print(diff_console_log, end='')
             # Wait a second
             time.sleep(1)
+    except kazoo.exceptions.NoNodeError:
+        return False, 'ERROR: VM has gone away.'
+    except:
+        return False, 'ERROR: Lost connection to Zookeeper node.'
 
     return True, ''