Macro error_once

macro_rules! error_once {
    (target: $target:expr, $($arg:tt)*) => { ... };
    ($($arg:tt)*) => { ... };
}

Logs a message once at the error level.

The log event will only be emitted once for each combinaison of target/arguments.

Logging at this level is disabled if the max_level_off feature is present.