Be clearer about what types we're catching.

[copybara-service[bot]]

Be clearer about what types we're catching.

In particular, this helps make clearer that none of these particular catch blocks catch arbitrary checked exceptions—and thus that none of them catch InterruptedException.

Also, I've removed some catch (Throwable t) blocks from FakeTimeLimiter. They could never be hit, since previous blocks had caught both Error and Exception (in the case of Callable) or RuntimeException (in the case of Runnable).

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