`context` and `context(true)` are treated as `context(false)`

[shepmaster]

Both of these should be compile-time errors.

#[derive(Debug, Snafu)]
enum Error {
    #[snafu(context(true))]
    Alpha
}

#[derive(Debug, Snafu)]
enum Error {
    #[snafu(context)]
    Alpha
}