Metadata viewer tweaks

- Don't show focus distance when it isn't usable (which is kind of weird, I'd have expected the Canon 6D + 24-105 to support this, but even the `exiv2 -pv` doesn't show anything useful)

- Add "mm" units for the focal length

- Spell out "ISO" in capital letters. I went as far as looking to the source code to see why ISO wasn't displayed by default before I realized that it's "iso", not "ISO".