add FieldDoesNotExist exception to __all__

So it will be available when you do:
    from mongoengine import *

mongoengine/errors.py

@@ -5,7 +5,8 @@ from mongoengine.python_support import txt_type
 
 __all__ = ('NotRegistered', 'InvalidDocumentError', 'LookUpError',
            'DoesNotExist', 'MultipleObjectsReturned', 'InvalidQueryError',
-           'OperationError', 'NotUniqueError', 'ValidationError')
+           'OperationError', 'NotUniqueError', 'FieldDoesNotExist',
+           'ValidationError')
 
 
 class NotRegistered(Exception):