Added Error to return type of async validator

[vjau]

This PR changes the Typescript definition.
When using an async validator, errors are either throwed or returned.
The initial typing only considered the case when Error where throwed. This PR add a union type including Error on the returned Promise.
The test is also modified by returning an Error

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[vjau]

Sorry, i have seen that there is already a pending PR addressing what this PR is doing. #58
However, #58 is more ambitious (i have actually tried to do what #58 try to do the whole day without success), i think this more limited fix could be merged for now.
Thank you.

[vjau]

I have splitted the test as requested

[mcollina]

lgtm

[climba03003]

LGTM.