Preserve own properties of Error instances across serialization/deserialization

[aron-cf]

Closes #87.

Today the wire format for Error only carries name, message, and (optionally) stack. Anything a user attaches to the error — code, details, a cause, the inner errors of an AggregateError — is lost the moment the error crosses an RPC boundary.

This PR extends the error expression with an optional fifth element, props. This uses the same serialization/deserialization as any other object.

Error.cause and AggregateError.errors are normally non-enumerable, so they are picked up explicitly by the encoder. On the receive side, the decoder constructs the error as before and then assigns each entry from props as an own enumerable property on the result.

let err = new Error("rate limited");
err.code = "E_LIMIT";
err.retryAfter = 30;
throw err;
// receiver sees: err.code === "E_LIMIT", err.retryAfter === 30

Properties that cannot be fully serialized are dropped.

Tests have been added to verify the serialization/deserialization of various different error cases and failure modes. There may be too many, happy to remove any if needed.

The error expression in protocol.md now documents the optional props element, the lossy-fallback semantics.

[changeset-bot]

🦋 Changeset detected

Latest commit: 9025285

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
capnweb	Patch

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commit: 9025285

[aron-cf]

I've reworked the code to skip any error properties that fail to serialize. I've done this in a pre-scan pass (that early exits when it hits a negative) rather than attempting to recover from a serialization error, from what I understand this removes the risk of leaking ReadableStreams at the cost of two passes. Let me know your thoughts.