Update lock managet app

Add keyword arguments. Sort imports. Move settings and test
literals to their own module.

Signed-off-by: Roberto Rosario <Roberto.Rosario@mayan-edms.com>

mayan/apps/lock_manager/exceptions.py

@@ -1,2 +1,5 @@
+from __future__ import unicode_literals
+
+
 class LockError(Exception):
-    pass
+    """Raised when trying to acquire an existing lock"""