Consider using str.format() methods so that if someone wants to override the error.* messages you don't get an error if the %s is missing new error message.
>>> 'some message' % 'x'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: not all arguments converted during string formatting
>>> 'some message'.format('x')
'some message'
>>>```
Consider using str.format() methods so that if someone wants to override the error.* messages you don't get an error if the %s is missing new error message.