feat: add error stubs for vault_router and TypeScript SDK

contracts/vault_router/src/error.rs

@@ -0,0 +1,11 @@
+#![allow(unused)]
+use soroban_sdk::contracterror;
+
+#[contracterror]
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+#[repr(u32)]
+pub enum VaultError {
+    InvalidTier     = 1,
+    BelowMinDeposit = 2,
+    LockNotExpired  = 3,
+}

sdks/typescript/src/errors.ts

@@ -0,0 +1,14 @@
+export class YieldLadderError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = 'YieldLadderError';
+  }
+}
+
+export class LockNotExpiredError extends YieldLadderError {
+  constructor() { super('Lock period has not expired yet'); this.name = 'LockNotExpiredError'; }
+}
+
+export class BelowMinDepositError extends YieldLadderError {
+  constructor() { super('Amount is below the minimum deposit'); this.name = 'BelowMinDepositError'; }
+}