Fix traversal for widget in multiwidgets that are added on the fly

Solves the use case of traversing a new widget that is not initialized by
default.
This mimics what happens when you click the add button.

Author: ale-rt

src/plone/z3cform/traversal.py

@@ -72,7 +72,7 @@ def traverse(self, name, ignored):
                     # part should be integer index in list, look it up
                     target = target.widgets[int(part)]
                 except IndexError:
-                    raise TraversalError("'" + part + "' not in range")
+                    target = target.getWidget(int(part))
                 except ValueError:
                     # HACK: part isn't integer. Iterate through widgets to
                     # find matching name. This is required for

src/plone/z3cform/traversal.txt

@@ -154,13 +154,16 @@ And traverse through to individual items.
     >>> age_widget.value
     '50'
 
-Out of range errors are LocationErrors
+Sometimes you want to traverse to a widget that does not yet exist,
+e.g. the one that is added when you click the add button:
 
     >>> traverser = get_traverser(Bar(),u"test-list-form")
     >>> age_widget = traverser.traverse('list_field.9', [])
-    Traceback (most recent call last):
-      File "<stdin>", line 1, in ?
-    zope.location.interfaces.LocationError: "'9' not in range"
+    Updating test form
+    >>> age_widget
+    <TextWidget 'form.widgets.list_field.9'>
+    >>> age_widget.value
+    ''
 
 Non-integer values are also LocationErrors