Change behaviour of `as_binary` when data is not present?

[martinjthompson]

For my use-case it would be useful for as_binary to raise an Exception if I ask for data which is not contained in the files I have added.

At the moment, if padding is None, it is converted to a default of 0xFF . I would request the following (which admittedly changes the API):

make the default parameters more explicit:

def as_binary(self,
                  minimum_address=None,
                  maximum_address=None,
                  padding=0xFF):

and then alter the behaviour of padding==None to raise a RuntimeException if there is any data requested which does not 'exist'. Would this be acceptable?