7.1The mental model

Imagine:

HTTP Listener
     ↓
Validate
     ↓
Salesforce Upsert
     ↓
Billing API
     ↓
Response

Salesforce throws:

SALESFORCE:CONNECTIVITY

Mule creates an error object and looks for the applicable handler.

Conceptually:

try {
    await salesforce.upsert();
}
catch (error) {
    ...
}

But Mule gives errors structured types rather than forcing you to parse exception strings.