Fragile error formatting yields "[object Object]" for non-Error throwables

[KruGoL]

Summary

When a thrown value is not an Error instance, the SDK's error-formatting idiom loses the actual error text and produces the useless string [object Object].

Reproduction

Creating a wallet with a nametag against the testnet aggregator while the API key is rejected (401):

Wallet creation with nametag failed: SphereError: Failed to mint nametag token: Submit failed: [object Object]
    at at.registerNametag
    at async at.create
    at async at.init

Root cause

1. The aggregator client (@unicitylabs/state-transition-sdk) throws JsonRpcNetworkError on a non-OK HTTP response. That class is not a subclass of Error and has no toString() — unlike its sibling JsonRpcDataError. It does carry useful fields: { name, status, message }.
2. sphere-sdk catches it with the idiom error instanceof Error ? error.message : String(error) (e.g. modules/payments/NametagMinter.ts:182). Since error instanceof Error is false, it falls to String(error) -> Object.prototype.toString() -> [object Object]. The real status: 401 and response body are discarded.

This idiom appears in ~50 places across ~25 source files, so every non-Error throwable that reaches one of them is mis-formatted.

Note: the 401 itself is a configuration issue (invalid testnet API key), not an SDK bug — but the bad formatting hides it from developers.

Proposed fix (sphere-sdk side)

• Add a formatError(err: unknown): string helper in core/errors.ts that extracts name / status / code / message from error-like objects and falls back to a JSON dump.
• Export it from core/index.ts and the main index.ts.
• Replace all instanceof Error ? x.message : String(x) call sites with formatError(x).
• Unit tests covering Error, JsonRpcNetworkError-shaped objects, JsonRpcDataError-shaped objects, strings, plain objects, null/undefined.

Out of scope (upstream)

JsonRpcNetworkError in @unicitylabs/state-transition-sdk should ideally extend Error (or add toString()). Tracked separately; the sphere-sdk fix makes the SDK resilient regardless.

[KruGoL]

Duplicate of #191 — already tracked there, and PR #192 (fix/issue-191-error-stringification) addresses it. Closing in favor of the existing issue/PR.