core: frontend: Improve axios middleware interface

Currently, someone using the middleware has to ignore backend offline error by putting a `if return` clause on the catch promise resolution, like that:

```typescript
back_axios({ ... })
.then((response) => { ... }
.catch((error) => {
if (error === backend_offline_error) { return }
...
```

It would be cool to have something like @patrickelectric suggested:

```typescript
back_axios({ ... })
.ignoreBackEndError()
.then((response) => { ... })
.catch((error) => { ... })
```