Fix inner audio handlers were unable to propagate errors when using SwitchAudioHandler #986

[dropout]

When using SwitchAudioHandlers, inner AudioHandler's were unable to propagate errors to SwitchAudioHandler

#986

Pre-launch Checklist

• [x] I read the [CONTRIBUTING.md] and followed the process outlined there for submitting PRs.
• [x] My change is not breaking and lands in minor branch OR my change is breaking and lands in major branch.
• [x] If I'm the first to contribute to the next version, I incremented the version number in pubspec.yaml according to the [pub versioning philosophy].
• [x] I updated CHANGELOG.md to add a description of the change (format: * DESCRIPTION OF YOUR CHANGE (@your-git-username)).
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I ran dart analyze.
• [x] I ran dart format.
• [x] I ran flutter test and all tests are passing.