Minimize the use of /* istanbul ignore */ comments

[mcmire]

Jest uses Istanbul under the hood to scan for and report on test coverage. At any point we can instruct Istanbul to exclude a section of code for consideration in the coverage report. However, long-term doing this is not ideal, and we should remove these directives:

• Many of the ignored lines/branches are either dead code, or represent important behaviors that are not being tested. We should delete dead code, and test all expected behaviours.
• The use of ignore comments should be reserved for conditions that are unreachable but still serve a purpose (e.g. type guards, certain validation steps, etc.).