Check if represented attributes are cached
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Improve existing cache checks in `/bluetooth/lib.rs`, and update Step comments in `script/dom` corresponding to this.
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It could be used to have mutable JSVal fields without GC barriers.
With the removal of that trait, MutHeap and MutNullableHeap can respectively
be replaced by MutJS and MutNullableJS.
WriteValue should return undefined
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Refactor Bluetooth error handling
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Replace the error messages with an enum in `net/bluetooth_thread.rs`. Rename `bluetooth_blacklist.rs` to `bluetooth_utils.rs` and put the error conversion in it.
With this the returned errors in DOM classes follow the specification.
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because there is no Web Bluetooth test API implementation yet.
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