Error on patches mentioning binary file differences

[sourcefrog]

From sourcefrog/cargo-mutants#391:

patch seems to return an error, or in some cases panic, on diffs that mention that a binary file changed. I guess #22 would change the panic into an error.

Some options:

1. Just drop these messages- would work for me, but maybe other clients want to know about them?
2. Maybe change the results to have some kind of enum indicating there was a change to a binary file? But this might be an API break to handle an edge case.
3. Add an option to control whether to drop or error?