getByWalletId now throws RecordNotFound

I propose to change a behavior of `getByWalletId` function: when a wallet is not found it should throw `RecordNotFound` rather then return `null`. When developer forgets to handle this situation by:
```js
if (!wallet) {
  // ...
}
```
we can be sure that `catch` block will catch it. Right now there are a few places where this function is used but `if (!wallet)` condition is not checked.