foo.js

fetch('https://min-api.cryptocompare.com/data/price?fsym=MXN&tsyms=BTC')
    .then(response => { 
        if(response.ok){
            return Promise.resolve(response);
        } else{
            return Promise.reject(new Error("No funciona"));
        }
    })
    .then(response => response.json())
    .then(data => {
        var valor = convertir(data.BTC, cantidad);
        desplegar(valor);
    })
    .catch(error => { console.log(`Error: ${error}`) });