on YAML error display the original error message

while the exception was present in the format arguments, 
it didn't have a placeholder in the actual error message
string that was being formatted. 

Note: str.format and the old style % have different behavior (python 2.6/2.7):

>>> '%s %s' % ('sdf', 'qwerty', 'this is the omitted third argument')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: not all arguments converted during string formatting
>>> '{0} {1}'.format('sdf', 'qwerty', 'this is the omitted third argument')
'sdf qwerty'