fix(evm-wallet-experiment): surface underlying twin errors in multi-twin wrapper

The `away-coordinator.transferNative` / `transferFungible` wrappers
(introduced in PR #939 when the coordinator was split) wrap per-twin
rejections as `throw new Error('All delegation twins failed', { cause: errors })`.
The kernel's RPC error serialization only propagates `Error.message`,
not `Error.cause`, so callers (including `run-delegation-twin-e2e.mjs`)
only see the generic "All delegation twins failed" — which loses the
actual reason (e.g. "Insufficient budget: requested 3, remaining 2").

Concatenate the cause messages into the wrapper's message text so the
specific per-twin rejection reason survives to the caller. Keep the
structured `cause` array intact for programmatic consumers.

Unblocks the "enforces cumulativeSpend locally" assertion in the
delegation-twin docker e2e test.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: sirtimid

packages/evm-wallet-experiment/src/vats/away-coordinator.ts

@@ -1811,7 +1811,14 @@ export function buildRootObject(
             errors.push(error);
           }
         }
-        throw new Error('All delegation twins failed', { cause: errors });
+        throw new Error(
+          `All delegation twins failed: ${errors
+            .map((cause) =>
+              cause instanceof Error ? cause.message : String(cause),
+            )
+            .join('; ')}`,
+          { cause: errors },
+        );
       }
       if (homeSection) {
         return E(homeSection).transferNative(to, amt);
@@ -1852,7 +1859,14 @@ export function buildRootObject(
             errors.push(error);
           }
         }
-        throw new Error('All delegation twins failed', { cause: errors });
+        throw new Error(
+          `All delegation twins failed: ${errors
+            .map((cause) =>
+              cause instanceof Error ? cause.message : String(cause),
+            )
+            .join('; ')}`,
+          { cause: errors },
+        );
       }
       if (homeSection) {
         return E(homeSection).transferFungible(token, to, amt);