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

[jbencin]

Describe the bug There is an unwrap() when processing the response from /v2/transactions in l1_submit_tx here

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