better error messages if option doesn't exists

tests/test_simple.py

@@ -0,0 +1,14 @@
+from json import loads
+from tiramisu_api import Config
+from pytest import raises
+
+
+def test_list_option():
+    filename = 'tests/data/boolean1.json'
+    with open(filename, 'r') as fh:
+        json = loads(fh.read())
+    #
+    config = Config(json)
+    opts = []
+    raises(AttributeError, "config.option('unknown').value.get()")
+    raises(AttributeError, "config.option('options.unknown').value.get()")