Use enum variants for errors

[Ploppz]

In the code, we often describe errors with &'static str, e.g. bail!("Read past boundary of given contents.");. I find this convenient when the code is unstable or under rewrite, as we have yet to decide on the enum variants of ErrorKind in err.rs.

At some point, we should go through the code and see what enum variants we need to have in ErrorKind, and use those instead of str, so as to accommodate for applications to do pattern matching on errors.

[s3bk]

closing with #25