[Chore]: reconsider signer behaviour related to Emily sBTC limits

[matteojug]

Chore - reconsider signer behaviour related to Emily sBTC limits

1. Description

The block observer fetches the limits from Emily, after processing the block but before fetching new requests from Emily. If we fail to get the limits, we skip loading new requests from Emily.

sbtc/signer/src/block_observer.rs

Lines 160 to 163 in dfddc97

	if let Err(error) = self.update_sbtc_limits().await {
	tracing::warn!(%error, "could not update sBTC limits");
	continue;
	}

In case of failures in getting the limits (Emily unreachable, breaking change in the limit format, ...), while we would not process new requests, already known requests in the database would still be processed using the last known limits.

This task is to re-evaluate if we want to keep this behaviour, or if we prefer to bail/default to conservative limits even if we have some previous known limits.