Expose parse errors via Dotenv::iter.

[jsirois]

Refactor dotenv::dotenv::Iter to expose parse errors for those
interested. The parse errors are structured as opaque strings to hide
the parsing implementation details (currently nom and nom's errors are
the internal currency).

This allows a consumer that cares about propagating errors in .env
files to use something like:

let env = dotenv::from_filename(".env")?;
let mut iter = env.iter();
while let Some((key, value)) = iter.try_next()? {
    if std::env::var(key).is_err() {
        std::env::set_var(key, value);
    }
}

Fixes #4

[jsirois]

Thanks for this! I did not know about non_exhaustive until this change and it is what allowed this PR's conservative approach of not breaking existing users / requiring a major semver bump.

[jsirois]

I assumed this duplicated line was unintentional (it was the only one in all the examples) and it got in the way of fixing tests to assert all expected values were observed. If this was intentional though, I can go back to work on handling this differently.