Consolidate `parse_labels()` errors

[ezraporter]

Description

This PR consolidates the two errors thrown by parse_labels() which we can't effectively distinguish and adds test cases to cover both missing commas and extra pipes.

Proposed Changes

List changes below in bullet format:

• Consolidate errors
• Add test cases

Issue Addressed

Closes #130

PR Checklist

Before submitting this PR, please check and verify below that the submission meets the below criteria:

• [x] New/revised functions have associated tests
• [x] New/revised functions that update downstream outputs have associated static testing files (.RDS) updated under inst/testdata/create_test_data.R
• [x] New tests that make API calls use httptest::with_mock_api and any new mocks were added to tests/testthat/fixtures/create_httptest_mocks.R
• [x] New/revised functions use appropriate naming conventions
• [x] New/revised functions don't repeat code
• [x] Code changes are less than 250 lines total
• [x] Issues linked to the PR using GitHub's list of keywords
• [x] The appropriate reviewer is assigned to the PR
• [x] The appropriate developers are assigned to the PR
• [x] Pre-release package version incremented using usethis::use_dev_version()

Code Review

This section to be used by the reviewer and developers during Code Review after PR submission

Code Review Checklist

• [ ] I checked that new files follow naming conventions and are in the right place
• [ ] I checked that documentation is complete, clear, and without typos
• [ ] I added/edited comments to explain "why" not "how"
• [ ] I checked that all new variable and function names follow naming conventions
• [ ] I checked that new tests have been written for key business logic and/or bugs that this PR fixes
• [ ] I checked that new tests address important edge cases

[ezraporter]

So just checking, we're not changing any logic just consolidating the flag that could be brought up for either case under one more generalized error message?

Yup I'm actually 95% sure the second check can't fire without triggering the first but I decided we might as well keep both in case there's some edge case we're missing.