Document error for non-existing code hash

frame/contracts/src/exec.rs

@@ -1029,13 +1029,13 @@ where
 	fn set_code_hash(&mut self, hash: CodeHash<Self::T>) -> Result<(), DispatchError> {
 		let top_frame = &mut self.top_frame_mut();
 		let prev_hash = top_frame.contract_info().code_hash.clone();
-		top_frame.contract_info().code_hash = hash;
 		if !Self::code_exists(&hash) {
 			return Err(DispatchError::CannotLookup)
 		}
+		top_frame.contract_info().code_hash = hash;
 		<ContractInfoOf<T>>::insert(top_frame.account_id.clone(), top_frame.contract_info());
-		increment_refcount::<Self::T>(hash);
-		decrement_refcount::<Self::T>(prev_hash);
+		increment_refcount::<Self::T>(hash)?;
+		decrement_refcount::<Self::T>(prev_hash)?;
 		Ok(())
 	}
 

frame/contracts/src/wasm/runtime.rs

@@ -828,6 +828,7 @@ define_env!(Env, <E: Ext>,
 	//
 	// - requested buffer is not within the bounds of the sandbox memory.
 	// - the buffer contents cannot be decoded as the required type.
+	// - code for specified hash does not exist
 	[__unstable__] seal_set_code_hash(ctx, code_hash_ptr: u32) -> ReturnCode => {
 		ctx.charge_gas(RuntimeCosts::SetCodeHash)?;
 		let code_hash: CodeHash<<E as Ext>::T> = ctx.read_sandbox_memory_as(code_hash_ptr)?;