Handling Errors Gracefully for the UI

@AuraEnabled
public static void saveOpportunity(Opportunity opp) {
    update opp; // if this throws, the raw DmlException message reaches the UI
}

1. @AuraEnabled — marks the method.
2. public static void saveOpportunity(Opportunity opp) { — the method.
3. update opp; — an unhandled DmlException here could surface a raw, technical message straight to the end user — something like a field-level API name or an internal validation-rule identifier, neither of which means anything to someone using the UI.
4. } — closes the method.

@AuraEnabled
public static void saveOpportunity(Opportunity opp) {
    try {
        update opp;
    } catch (DmlException e) {
        AuraHandledException ex = new AuraHandledException('Could not save the opportunity. Please check the required fields and try again.');
        ex.setMessage('Could not save the opportunity. Please check the required fields and try again.');
        throw ex;
    }
}

1. @AuraEnabled — marks the method.
2. public static void saveOpportunity(Opportunity opp) { — the method.
3. try { — catching the real, technical exception (Module 13's try/catch).
4. update opp; — the risky operation.
5. } catch (DmlException e) { — catches the raw failure.
6. AuraHandledException ex = new AuraHandledException('Could not save the opportunity. Please check the required fields and try again.'); — AuraHandledException is specifically designed to carry a message safely across the Apex-to-LWC boundary.
7. ex.setMessage('Could not save the opportunity. Please check the required fields and try again.'); — the message must be set both in the constructor and via setMessage() due to a well-known Apex quirk with how this specific exception type serializes its message.
8. throw ex; — re-throws this UI-safe version with a clear, human-readable message instead.
9. } — closes the catch.
10. } — closes the method.

This closes out the same discipline Module 30's "review and refactor" lesson applied to Case routing — validated input, bulk-safe logic, deliberate sharing, and errors handled gracefully — now applied specifically at the point where Apex hands control back to a UI a real user is looking at. A method exposed to an LWC deserves every one of those checks, since it's directly reachable by anyone using the application.