[trace] introduce trace failed to Ext (#11019)

* [trace] add trace_failed to Ext, manage stack of trace data

* [evm] call trace_failed only if self.do_trace

* [evm] add a comment about do_trace set to true

* [vm] improve the doc in trace_prepare_execute

* [trace] add the bounds check back

ethcore/vm/src/ext.rs

@@ -164,8 +164,12 @@ pub trait Ext {
 	fn trace_next_instruction(&mut self, _pc: usize, _instruction: u8, _current_gas: U256) -> bool { false }
 
 	/// Prepare to trace an operation. Passthrough for the VM trace.
+	/// For each call of `trace_prepare_execute` either `trace_failed` or `trace_executed` MUST be called.
 	fn trace_prepare_execute(&mut self, _pc: usize, _instruction: u8, _gas_cost: U256, _mem_written: Option<(usize, usize)>, _store_written: Option<(U256, U256)>) {}
 
+	/// Trace the execution failure of a single instruction.
+	fn trace_failed(&mut self) {}
+
 	/// Trace the finalised execution of a single instruction.
 	fn trace_executed(&mut self, _gas_used: U256, _stack_push: &[U256], _mem: &[u8]) {}