Refactor error report deserialisation

[fractalwrench]

Goal

The ErrorReader class is hard to read in places which may have contributed to #418. This changeset attempts to improve readability by extracting methods so that each has a single responsibility.

Changeset

Breadcrumbs, stackframes, and threads are all serialised as a JSON array in the error report. The current implementation completes something like the following steps in a single method:

// 1. old scheme
void readBreadcrumbs() {
    // create an array
    // iterate around array objects
    // deserialise a breadcrumb
    // end an array
}

The step of deserialising individual JSON objects has now been extracted to a separate method, like thus:

// 2. new scheme
void readBreadcrumbs() {
    // create an array
    // iterate around array objects
    breadcrumbs.add(readBreadcrumb())
    // end an array
}

Breadcrumb readBreadcrumb() {
    // deserialise a breadcrumb
}

This ensures each method only has a single responsibility and improves readability of the code.

Tests

Ran existing tests, and performed testing as detailed in #418.

[bengourley]

Looks good. What's up with the PR CI failure, flakey CI?

[fractalwrench]

Yep, down to flaky CI - I've restarted the job.