fix: throw when adding sub account owner fails

[Snowmanzx]

sendRequestToSubAccountSigner swallowed errors when handleAddSubAccountOwner failed.

The catch block did return standardErrors.provider.unauthorized(...). That factory returns an EthereumProviderError, it does not throw. So the async function resolved with the error object as its value instead of rejecting. The only caller awaits the result and returns it so its catch never runs and the dApp gets the error object back as if it were a valid result.

Changed return to throw, which matches how the rest of the function reports errors (assertPresence throws the same error type above).

Added a test in the auto sub account block that makes handleAddSubAccountOwner reject and checks that request rejects.

Closes #326

[cb-heimdall]

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