(Proposal) implement db.one()

[jniles]

Inspired by the pg-promise library, I propose that we implement a convenience function db.one(sql, params, tableName). db.one() will call db.exec() internally, but will ensure that only one result is returned.

Proposed behavior:

1. If the db.exec() call returns no rows, throw a new NotFound() error with the tableName in the description.
2. If the db.exec() call returns exactly one row, return rows[0]
3. If the db.exec() call returns multiple rows, log a warning and return rows[0].

For example:

// on the accounts controller
function detail(req, res, next) {
  db.one('SELECT * FROM account WHERE id = ?', [req.params.id], 'account')
  .then(row => res.status(200).json(row))
  .catch(next)
  .done();
}

// in the purchase orders controller
function detail(req, res, next) {
  db.one(
    `SELECT p.uuid, p.date, p.supplier_uuid, pi.cost 
     FROM purchase p JOIN purchase_item pi ON p.uuid = pi.purchase_uuid 
     WHERE p.uuid = ?`, [req.params.uuid], 'purchase'
  )
  .then(row => res.status(200).json(row))
  .catch(next)
  .done()
}

[jniles]

I'll take this one to clean up the controller code for #694.