Return a more descriptive error message

When socket_file is not found we return:
UNIX socket file was not set

which is not descriptive at all when the file is not found.

Let's fix that by catching the case where the file is not present
and return a better message

haproxyadmin/haproxy.py

@@ -114,7 +114,9 @@ def __init__(self,
             for _file in glob.glob(os.path.join(socket_dir, '*')):
                 if is_unix_socket(_file) and connected_socket(_file, timeout):
                     socket_files.append(_file)
-        elif (socket_file and is_unix_socket(socket_file) and
+        elif (socket_file and not os.path.exists(socket_file)):
+            raise ValueError("{} UNIX socket file was not found".format(socket_file))
+        elif (socket_file and os.path.exists(socket_file) and is_unix_socket(socket_file) and
               connected_socket(socket_file, timeout)):
             socket_files.append(os.path.realpath(socket_file))
         else: