Using result type to reflect scanning error

[athanclark]

This system already has a ScannerError that looks like it was built to be extended - I just added a ScannerError::String data constructor, and passed the call to error!(...) into ScannerError::String(format!(...)) to allow end-users the ability to decide how they want to handle the error. I also removed the process kill line.

[delfanbaum]

Thanks for catching this! The CI is failing because of an issue with a feature dependency, so given that I'm about to fix that in the release I'm prepping, we can just merge this in and I'll do what little fiddling I'm going to do on the (rebased) release branch.

Just FYI, one crate-idiomatic thing I'm going to change is to make the error less generic so that we can match on it if that's ever needed -- just to make it align more closely with the other error structs in the repo.

The one code feedback thing I would note is that since that function is just a big match, you don't need the explicit return (since the other arms are returning, essentially Ok(token), so that line could instead be written:

_ => Err(ScannerError::String(format!("Invalid headling level: line {}", self.line)))

In any case, thanks for the contribution!