[nextest-runner] simplify reporter error handling

[sunshowers]

Trying to handle reporter errors in the main runner loop is a bit of a mess. Instead, handle them at a higher level and use a oneshot channel to indicate that an error has occurred. This fits in quite naturally with the rest of the event-driven system.

Also change "canceled" to "cancelled" for consistency.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 69.07216% with 30 lines in your changes missing coverage. Please review.

Project coverage is 79.79%. Comparing base (f1966f7) to head (bbede97). Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
nextest-runner/src/runner.rs	68.08%	30 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1836 +/- ## ========================================== - Coverage 79.82% 79.79% -0.03% ========================================== Files 80 80 Lines 20362 20376 +14 ========================================== + Hits 16253 16260 +7 - Misses 4109 4116 +7 ```

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