Node would crash if invalid JSON returned from `/v2/transactions` endpoint

**Describe the bug**
There is an `unwrap()` when processing the response from `/v2/transactions` in `l1_submit_tx` [here](https://github.com/hirosystems/stacks-subnets/blob/c0287d39c376d1f0a984645f6b9f1796892f70ec/testnet/stacks-node/src/burnchains/l1_events.rs#L214)

**Expected behavior**
The application should never crash on incorrect/malicious network input, as this is a potential denial-of-service vector

**Additional context**
`unwrap()` should be replaced with `?`, and if necessary the appropriate `From` trait needs to be implemented on `subnet_node::burnchains:Error` to convert the error