exception when attempting to read a non-existing file

```
What steps will reproduce the problem?
1. regdiff.exe nonexistent.file som-registry-key
2. boom!
3.

What is the expected output? What do you see instead?
an error message

```

Original issue reported on code.google.com by `gerson.k...@gmail.com` on 11 Mar 2014 at 12:02
