Surface actual IDB errors instead of raw error events

[mlaw-]

Previously, when an IndexedDB operation failed, callers received the raw
IDBRequest error event rather than the underlying DOMException. This made
error type detection awkward — callers had to navigate err.target.error.name
instead of simply checking err.name or err instanceof DOMException.

Changes

WrapRequest — rejects with req.error (the DOMException) instead of the raw
event. Falls back to the event only if req.error is null.

Transaction onerror/onabort — passes this._trans.error directly to
transactionFailed rather than building a string from it, so the real
DOMException propagates through the transaction's completion promise.

TransactionLockHelper.transactionFailed — accepts string | Error so typed
error objects pass through unchanged.

open() error handler — updated to work with the DOMException directly:
VersionError detection uses err instanceof DOMException && err.name === "VersionError" instead of inspecting event properties; UpgradeCallback error
fields now populate from err.name/err.message.

Result

Callers can now detect specific IDB errors:

try {
await transaction.put("store", item);
} catch (err) {
if (err instanceof DOMException && err.name === "QuotaExceededError") {
// handle quota exceeded
}
}

Tests

• WrapRequest rejects with the actual DOMException (not the event)
• WrapRequest falls back to the raw event when req.error is null
• put propagates the DOMException to the caller

[Thomas Tay (thomastay)]

lgtm