Fix FallbackProvider getAnyResult() with errors

[staltz]

Problem

When submitting a transaction (i.e. broadcastTransaction) with a FallbackProvider with 3 sub-providers (each with weight 2) and a quorum configured at 3, the _perform will use getAnyResult to pluck the result from each of the sub-providers.

The way getAnyResult works (especially when checkQuorum is not used due to weights being smaller than the quorum number), means that it will pick the first result, regardless if it's an error or not.

In the context of broadcasting transactions, there are cases where one provider gets the transaction request, while other providers that are slower will error with "already known", which in turn causes Ethers to produce

Error: could not coalesce error (error={ "code": -32000, "message": "already known" }

See issue #4186

Solution

In getAnyResult, give preference to early-returning non-error results.

[ricmoo]

This has been fixed in v6.9.0.

Please let me know if you find any other issues; there is also a robust FallbackProvider test suite to help identify and create reproducible examples, which ensure no regressions.

Thanks! :)