Catch unhandled promise rejections - #196

Summary

(If you have not already please refer to the contributing guideline as described
here)

• Tell us about the problem your pull request is solving.

When use transaction hooks, if a transaction failed in either establishing or execution phase, an unhandled rejection error happens. The reason is that knex.transaction returns a promise and no error handler is attached to the promise within a turn of the event loop. Unhandled promise rejections are deprecated and it will terminate Node.js process with a non-zero exit code, better to catch them and throw a customized error instead.

Here is a simplified example for your reference:

// mock async function to throw an error
const asyncError = fn => {
  return Promise.resolve()
    .then(() => { throw new Error('error'); })
    .then(fn);
}

// hook list
const arr = [
  hook => {
    hook.params.test1 = 1;
    return hook;
  },
  hook => Promise.resolve(hook).then(hook => {
    hook.params.test2 = 2;
    return hook;
  }),
  // hook passes down error occurred in `asyncError`
  hook => new Promise((resolve, reject) => {
    asyncError(obj => resolve(obj)).catch(err => reject(err));
  })
  .then(obj => {
    hook.params.success = { obj };
    return hook;
  })
  .catch(err => {
    throw err;
  }),
  // hook neglects error occurred in `asyncError`,
  // comment the hook above and uncomment this one to see an unhandled rejection
  // hook => new Promise(resolve => {
  //   asyncError(obj => {
  //     hook.params.failure = { obj };
  //     return resolve(hook);
  //   })
  //   .catch(err => {
  //     console.log('catch 2');
  //     throw err;
  //   })
  // })
]

// simulate hook execution
arr.reduce((promise, fn) =>
  promise.then(hook => {
    console.log(hook);
    return hook === 'SKIP' ? 'SKIP' : fn(hook);
  }), Promise.resolve({ params: {} }))
  .then(result => console.log(result))
  .catch(err => {
    console.error(err);
  });

• Are there any open issues that are related to this?
  No.
• Is this PR dependent on PRs in other repos?
  No.

If so, please mention them to keep the conversations linked together.

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