add account not found exception

- update balance in AccountController with onErrorResume
- update AccountService
  - update getForUpdateById with switchIfEmpty
  - update getById with switchIfEmpty
- add AccountNotFoundException

src/main/kotlin/ltd/lulz/controller/AccountController.kt

@@ -39,6 +39,6 @@ class AccountController(
         @PathVariable account: UUID,
     ): Mono<Account.Response> = accountService.getById(account)
         .map { it.toResponse() }
-        .switchIfEmpty(Mono.error(ResponseStatusException(NOT_FOUND)))
+        .onErrorResume { Mono.error(ResponseStatusException(NOT_FOUND)) }
         .doOnError { log.debug { "account $account not found for balance" } }
 }