docs: Standardise error messages format

Returned errors per service should have the following format:

```
fmt.Errorf("/: ")
```

for example:

```
fmt.Errorf("header/store: no valid headers were given")
```

### What should be done

1. Grep code for `fmt.Errorf` and `errors.New`
2. Implement if it's not following this style `"/: "`
3. 🚢

[Ref](https://github.com/celestiaorg/celestia-node/pull/195#discussion_r749234414).