When DSN is missing, lock silently fails

[IlyaSemenov]

If DSN is not provided at all (e.g. because of the misconfigured environment), advisory-lock silently fails to lock:

> require('advisory-lock').default()('test').tryLock().then(locked => console.log('locked', locked)).catch(console.error)
> locked false

With proper DSN it works:

> require('advisory-lock').default('postgresql://localhost/test')('test').tryLock().then(locked => console.log('locked', locked)).catch(console.error)
> locked true

With wrong DSN it fails (as expected):

> require('advisory-lock').default('postgresql://localhost/no_such_db')('test').tryLock().then(locked => console.log('locked', locked)).catch(console.error)
> { error: database "no_such_db" does not exist ...

Expected behaviour

With undefined DSN, an error is thrown.

[olalonde]

Oops, could you send a PR?