Handle gateway response errors

[fracek]

I noticed that when the gateway returns a "too many requests" error, the error I get from the SequencerGatewayProvider is a deserialization error (because the gateway returns an html page). I propose to add a RateLimit error to ProviderError to cover this case.

[xJonathanLEI]

Yeah makes sense to me, as long as the gateway returns a 429 status code properly.

[xJonathanLEI]

Our current test setup doesn't include HTTP status code or headers, making something like this hard to test as we can't just use the real gateway. So I will make changes to the test setup first before working on this.