Make error::Kind public

[levkk]

Make error::Kind public and provide a getter. This allows lib/app code to handle different kind of errors, e.g.

match err.kind() {
    Kind::Column(name) => panic!("You forgot to include column {} into the query", name),
    Kind::Parameter(_, _) => panic!("You forgot to pass a parameter to the prepared statement"),
    kind => panic!("{:?}", kind),
}

I documented errors I know about, but the ones I don't know what they do I left undocumented, hiding the doc warning.

[BaptisteRoseau]

Also made it here #1138 with a few test cases but it seems like it won't be merge any time soon..

[Aceeri]

This is something I also would love, I've just ended up doing err.to_string() and re-parsing it which is pretty counterproductive.

[paolobarbolini]

These usizes are very ambiguous. Is this the index of the column? We should document it.