fix: throw error instead of string

[justsml]

Fixes #9283

Description

This is a minor issue around error visibility.

Here's how Bluebird's warning about the issue shows up:

Warning: a promise was rejected with a non-error: [object String]

To fix this, rej(string) must be called with a new Error(string) instead.

[pieh]

Thanks @justsml!

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