Macro ensure 

macro_rules! ensure {
    ($cond:expr, $err:expr) => { ... };
}

Ensure a condition is true, otherwise return an error.

Example

use dbrest::ensure;
use dbrest::Error;

fn validate(x: i32) -> Result<(), Error> {
    ensure!(x >= 0, Error::InvalidQueryParam {
        param: "x".to_string(),
        message: "must be non-negative".to_string(),
    });
    Ok(())
}