Make rejected promise always yield error instance

[ncjones]

This change makes all request errors compatible with VError which expects any cause to be an instance of Error.

For example:

try {
  return await dynamics.retrieveRequest({ collection, key });
} catch(e) {
  if (e.status == '404') {
    throw new VError({ cause: e, name: 'InvalidMessage' }, 'Equipment not found %j', { key });
  }
  throw new VError(e, 'Failed to get equipment %j', { key });
}

Without this change the VError constructor throws "cause is not an Error", masking the error. I think this issue should be fixed in VError too (TritonDataCenter/node-verror#66).

[coveralls]

Coverage increased (+0.005%) to 96.221% when pulling c5b2e85 on ncjones:real-error-obj into 80cf08a on AleksandrRogov:master.

[AleksandrRogov]

Thank you for this PR, I see one issue here though: what if the CRM error object has inner error object? For example here. I guess we'll need to reassign keys recursively. Do you know if that "modern" spread operator works the latter way? If so, we need to use that syntax and I will add compilation commands to scale js version down as you suggested.

[ncjones]

The nested objects will be copied by reference so it should just work as it is. AFAIK the spread operator can only be used when creating object literals. Object.assign could be used here for brevity though.