fix: failed requests reach the page with an error code and the node's data (closes #533)

A page's request that the node refuses now reaches it with the node's
own JSON-RPC code and data: a proxied read such as a reverting eth_call,
a transaction whose gas estimate reverts, and a failed broadcast. Every
other refusal carries a code too: 4100 when there is no address, -32602
when the site asks for two different gas limits or a transaction type
this wallet does not sign, and -32603 for a failure inside the wallet.

nodeError() in approvalVerify.js reads the node's error out of what
ethers throws, as failureTexts() already did inline.

Model: opus-5-5

README.md

@@ -2200,8 +2200,10 @@ view would leave a wallet one click from deletion.
   one shown and signed in place of the node's estimate, but the node still
   estimates the transaction with that limit, as the check that it neither
   reverts nor runs out of gas; a request that gives both with different values
-  is refused. A request that cannot be populated — unreachable node, reverting
-  gas estimate — opens no window and is failed back to the site. Only one
+  is refused with EIP-1474 code `-32602`. A request that cannot be populated —
+  unreachable node, reverting gas estimate — opens no window and is failed back
+  to the site, with the node's own error code and data when the node refused it,
+  so the site can decode a revert, and with `-32603` otherwise. Only one
   transaction approval exists at a time: populating fixes the nonce, so a second
   `eth_sendTransaction` arriving while one is unanswered is refused with
   EIP-1193 code `-32002` rather than being populated at the same nonce. It opens