feat: add ETIMEDOUT error code for timeout error

[diorahman]

Added an error code ETIMEDOUT to easily identify timeout error at caller site. Previously, we need to do checking with e.g. indexOf('Timed out') >= 0.

This is useful when we need to abort or cancel the underlying process. The following contrived example tries to depict the usefulness of having ETIMEDOUT as error.code for when breaker timed out.

const circuitBreaker = require('./');

let handle;

function asyncTimeout () {
  return new Promise((resolve) => {
    handle = setTimeout(() => {
      console.log('timeout!');
      resolve();
    }, 1000);
  });
}

const breaker = circuitBreaker(asyncTimeout, { timeout: 500 });
breaker.fire()
  .then(console.log)
  .catch((err) => {
    // this is very naive but, imagine we have access to abort function for the underlying process
    // without this, the asyncTimeout will be fired.
    if (err.code === 'ETIMEDOUT') { // rather than e.message.indexOf('Timed out') === 0
      clearTimeout(handle);
    }
    console.error(err);
  });

[coveralls]

Coverage increased (+0.003%) to 99.278% when pulling 1f23114 on diorahman:timeout-error-code into 727e803 on bucharest-gold:master.

[lance]

@diorahman thanks for the contribution

[diorahman]

@lance 🎉 BTW, when is the next release?

[lance]

@diorahman today 👍 https://github.com/bucharest-gold/opossum/blob/master/CHANGELOG.md#110-2017-06-06