Macro unsupported_client_cell 

macro_rules! unsupported_client_cell {
    ($msg:expr) => { ... };
    ($msg:expr, $hopnum:expr) => { ... };
    (@ $msg:expr, $hopnum_display:expr) => { ... };
}

Return a CircProto error for the specified unsupported cell.

This error will shut down the reactor.

Note: this is a macro to simplify usage (this way the caller doesn’t need to .map() the result to the appropriate type)