let field choose if an attribute has to exist and not raise a key error

src/z3c/configurator/README.txt

@@ -117,7 +117,7 @@ The value must exist and be valid:
   >>> configurator.configure(something, {'foo': u'my value'})
   Traceback (most recent call last):
   ...
-  KeyError: 'bar'
+  RequiredMissing
 
   >>> something = Something()
   >>> configurator.configure(something, {'foo': u'my value', 'bar': 1})

src/z3c/configurator/configurator.py

@@ -98,7 +98,7 @@ def __init__(self, context):
 
     def verify(self, data):
         for name, field in zope.schema.getFields(self.schema).items():
-            field.validate(data[name])
+            field.validate(data.get(name))
 
     def __call__(self, data):
         raise NotImplemented