[#180] Test case for invalid design doc uri

And using the new style str formatter.

couchapp/clone_app.py

@@ -21,14 +21,12 @@ def clone(source, dest=None, rev=None):
     """
     Clone an application from a design_doc given.
 
-    :attr design_doc: dict, the design doc retrieved from couchdb
-    if something was wrong.
+    :param source: the http/https uri of design document
     """
-
     try:
         dburl, docid = source.split('_design/')
     except ValueError:
-        raise AppError("%s isn't a valid source" % source)
+        raise AppError("{0} isn't a valid source".format(source))
 
     if not dest:
         dest = docid

tests/test_clone_app.py

@@ -0,0 +1,17 @@
+# -*- coding: utf-8 -*-
+
+from couchapp.clone_app import clone
+from couchapp.errors import AppError
+
+from mock import patch
+from nose.tools import  raises
+
+
+@raises(AppError)
+def test_invalid_source():
+    '''
+    Test case for clone(invalid_source)
+
+    If a source uri do not contain ``_design/``, it's invalid.
+    '''
+    clone('http://foo.bar')