refactor: remove dead code for error handling

[nfischer]

This removes some dead code which does some error handling. These error cases should be obsolete now that we have tests which parse the full brothers.js file to ensure we can parse successfully.

This also adds /* istanbul ignore next */ statements for some code which is not testable: either because it is for error cases which are not expected, or because it is for browser-only code (tests run in NodeJS, so they won't ever be able to cover browser-only code).

Test: npm test Test: npm start (verify that this runs OK in the browser)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 72.72%. Comparing base (8a4331e) to head (24a135a).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #44 +/- ## =========================================== + Coverage 60.13% 72.72% +12.59% =========================================== Files 2 2 Lines 148 121 -27 =========================================== - Hits 89 88 -1 + Misses 59 33 -26 ```

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