Improve error messages in Coins::sub()

[sushiwushi]

This PR replaces the StdError::overflow() error with more informative error in the sub() function of the Coins struct.

The new errors display the amount of tokens wanted and the amount of tokens available, which will be helpful for developers who forget to mint tokens before sending native tokens in cw_multi_test (such as, me).

#[test]
fn overflow_error_not_enough_bal() {
    let mut app = App::default();
    let contract_box = ContractWrapper::new(
        crate::contract::execute,
        crate::contract::instantiate,
        crate::contract::query,
    );
    let code_id = app.store_code(Box::new(contract_box));
    let init_msg = crate::msg::InstantiateMsg {};
    let owner = "owner";
    let owner_addr = Addr::unchecked(owner);

    let funds = Coin {
        denom: DENOM.to_string(),
        amount: Uint128::new(1000),
    };

    // app.sudo(cw_multi_test::SudoMsg::Bank(
    //     cw_multi_test::BankSudo::Mint {
    //         to_address: owner.to_string(),
    //         amount: vec![funds.clone()],
    //     },
    // ))
    // .unwrap();

    app.instantiate_contract(code_id, owner_addr, &init_msg, &[funds], "dojo", None)
        .unwrap();
}

Error:

Caused by:
    0: Overflow: Cannot Sub with 0 and 1000
    1: Cannot Sub with 0 and 1000'

[chipshort]

Sorry for the long wait. Are you still interested in this?

[chipshort]

It's more advisable to use the StdError::generic_err constructor than constructing the variant directly. This code would fail to compile with the cosmwasm-std backtraces feature enabled.

[chipshort]

One small problem I see with this is that this makes it a bit more cumbersome to handle the error and makes an unnecessary allocation in that case.
With this change, you have to do something like:

if let Err(StdError::GenericErr { msg, .. }) = result {
    if msg.starts_with("Insufficient balance") {
        // handle
    }
}

as opposed to the current:

if let Err(StdError::Overflow { .. }) = result {
    // handle
}

But I think that's a bit of a niche case, so I am fine with this tradeoff for now.
Ideally, this would have it's own error type that converts into StdError (but that's a breaking change).