Macro tectonic::tt_warning

macro_rules! tt_warning {
    ($dest:expr, $( $fmt_args:expr ),*) => { ... };
    ($dest:expr, $( $fmt_args:expr ),* ; $err:expr) => { ... };
}

Report a formatted warning message to the user.

An Error object may be provided, in which case it will be shown to the user as well.