Better error when vault doesn't have enough funds

The spl_token transfer cpi instruction just says "error code: 1", which
is too opaque.

programs/mango-v4/src/error.rs

@@ -29,6 +29,8 @@ pub enum MangoError {
     NoFreePerpPositionIndex,
     #[msg("serum3 open orders exist already")]
     Serum3OpenOrdersExistAlready,
+    #[msg("bank vault has insufficent funds")]
+    InsufficentBankVaultFunds,
 }
 
 pub trait Contextable {

programs/mango-v4/src/instructions/flash_loan3.rs

@@ -82,6 +82,16 @@ pub fn flash_loan3_begin<'key, 'accounts, 'remaining, 'info>(
 
         // Transfer the loaned funds
         if *amount > 0 {
+            // Provide a readable error message in case the vault doesn't have enough tokens
+            if token_account.amount < *amount {
+                return err!(MangoError::InsufficentBankVaultFunds).with_context(|| {
+                    format!(
+                        "bank vault {} does not have enough tokens, need {} but have {}",
+                        vault_ai.key, amount, token_account.amount
+                    )
+                });
+            }
+
             let transfer_ctx = CpiContext::new(
                 ctx.accounts.token_program.to_account_info(),
                 token::Transfer {

programs/mango-v4/src/instructions/serum3_place_order.rs

@@ -1,4 +1,4 @@
-use crate::error::MangoError;
+use crate::error::*;
 
 use crate::serum3_cpi::load_open_orders_ref;
 use crate::state::*;
@@ -209,6 +209,20 @@ pub fn serum3_place_order(
     let before_base_vault = ctx.accounts.base_vault.amount;
     let before_quote_vault = ctx.accounts.quote_vault.amount;
 
+    // Provide a readable error message in case the vault doesn't have enough tokens
+    let (vault_amount, needed_amount) = match side {
+        Serum3Side::Ask => (before_base_vault, max_base_qty),
+        Serum3Side::Bid => (before_quote_vault, max_native_quote_qty_including_fees),
+    };
+    if vault_amount < needed_amount {
+        return err!(MangoError::InsufficentBankVaultFunds).with_context(|| {
+            format!(
+                "bank vault does not have enough tokens, need {} but have {}",
+                needed_amount, vault_amount
+            )
+        });
+    }
+
     // TODO: pre-health check
 
     //

programs/mango-v4/src/instructions/token_withdraw.rs

@@ -99,6 +99,16 @@ pub fn token_withdraw(ctx: Context<TokenWithdraw>, amount: u64, allow_borrow: bo
         // Update the bank and position
         let position_is_active = bank.withdraw_with_fee(position, amount_i80f48)?;
 
+        // Provide a readable error message in case the vault doesn't have enough tokens
+        if ctx.accounts.vault.amount < amount {
+            return err!(MangoError::InsufficentBankVaultFunds).with_context(|| {
+                format!(
+                    "bank vault does not have enough tokens, need {} but have {}",
+                    amount, ctx.accounts.vault.amount
+                )
+            });
+        }
+
         // Transfer the actual tokens
         let group_seeds = group_seeds!(group);
         token::transfer(