Exit properly on exclude-file error

[DimitriPapadopoulos]

If the exclude-file argument cannot be read, catch the exception and exit properly with a meaningful error message.

Fixes #3687.

[larsoner]

Worth adding some test that these can actually be hit? Makes sure that things like e.strerror actually work in the strings

[DimitriPapadopoulos]

@larsoner Do you think we should exit when a "secondary" file argument is missing, or just ignore the missing file as suggested in #3687 (comment)?

[larsoner]

Ahh for an --exclude I think it's okay if the file is missing, it shouldn't lead to an error