4. Errors, require, revert, assert and events

error Unauthorized(address caller);
error InsufficientFunds(uint256 needed, uint256 available);

function pay(uint256 amount) external {
    require(amount > 0, "Zero amount");                         // input validation (string costs more gas)
    if (balance[msg.sender] < amount) revert InsufficientFunds(amount, balance[msg.sender]);   // custom error (0.8.4+), cheapest
    assert(totalSupply >= balance[msg.sender]);                 // invariants that should NEVER fail (Panic error)
}

// try/catch for external calls
try oracle.latestPrice() returns (uint256 p) { price = p; }
catch Error(string memory reason) { emit OracleFailed(reason); }
catch (bytes memory) { emit OracleFailed("unknown"); }