Use Error instead of literal for promise rejection

[prsantos-com]

Changes

• Replaced literal with an instance of the Error class in promise rejection where applicable.
• Added the eslint rule prefer-promise-reject-errors to prevent this type of issue in the future. I set it to warn instead of error, as there was one promise rejection that used an object instead of a literal and fixing that would mean creating a custom error, which I feel is beyond the scope of this change.

Issues

None.