Catch OSError exceptions

If a user passes a socket file that does not exist then the
is_unix_socket will raise an exception :
➜ haproxytool acl -F /var/lib/haproxy/statss -s /etc/haproxy/block_traffic_header_names.list
Traceback (most recent call last):
  File "/home/pavlos/.virtualenvs/haproxy/bin/haproxytool", line 8, in <module>
    sys.exit(main())
  File "/home/pavlos/.virtualenvs/haproxy/lib/python3.9/site-packages/haproxytool/cli.py", line 41, in main
    call_main(sub_cmd)
  File "/home/pavlos/.virtualenvs/haproxy/lib/python3.9/site-packages/haproxytool/acl.py", line 107, in main
    hap = haproxy_object(arguments)
  File "/home/pavlos/.virtualenvs/haproxy/lib/python3.9/site-packages/haproxytool/utils.py", line 57, in haproxy_object
    hap = haproxy.HAProxy(socket_file=arguments['--file'],
  File "/home/pavlos/.virtualenvs/haproxy/lib/python3.9/site-packages/haproxyadmin/haproxy.py", line 117, in __init__
    elif (socket_file and is_unix_socket(socket_file) and
  File "/home/pavlos/.virtualenvs/haproxy/lib/python3.9/site-packages/haproxyadmin/utils.py", line 157, in is_unix_socket
    mode = os.stat(path).st_mode
FileNotFoundError: [Errno 2] No such file or directory: '/var/lib/haproxy/statss'

Let's avoid this situation by catching OSError class exceptions.

haproxyadmin/utils.py

@@ -154,7 +154,10 @@ def is_unix_socket(path):
     :type path: ``string``
     :rtype: ``bool``
     """
-    mode = os.stat(path).st_mode
+    try:
+        mode = os.stat(path).st_mode
+    except OSError:
+        return False
 
     return stat.S_ISSOCK(mode)