Before this change, if a playback error is thrown the test fails with a
timeout and no additional info:
```
java.util.concurrent.TimeoutException
at androidx.media3.exoplayer.e2etest.WebvttPlaybackTest.stallPlayerUntilCondition(WebvttPlaybackTest.java:361)
```
After this change, the test failure includes a much more useful stack
trace, e.g. from 0352db9a37:
```
Caused by: java.lang.IllegalStateException: Legacy decoding is disabled, can't handle text/vtt samples (expected application/x-media3-cues).
at androidx.media3.common.util.Assertions.checkState(Assertions.java:100)
at androidx.media3.exoplayer.text.TextRenderer.assertLegacyDecodingEnabledIfRequired(TextRenderer.java:587)
at androidx.media3.exoplayer.text.TextRenderer.onStreamChanged(TextRenderer.java:210)
```
PiperOrigin-RevId: 634672138
282a944eb4