Improve handling of Parsec handle errors and mocked data

[mmmarcos]

From @Max-7 #8483 (comment)

I'll have to do a big pass on everything related to how we mock data and how to handle these kind of errors once we got the web bindings.

Related to these kind of code structure found in client that can be problematic in a web-context:

const handle = getParsecHandle();
if (handle !== null && !needsMocks()) {
     // normal case
} else { 
    // error & mock case 
}