Refactor line check

Recreational refactoring and fighting against a **C** mark on https://codeclimate.com/github/envato/double_entry/DoubleEntry::LineCheck

Next step: Extract methods into a `LineChecker` *Service* Model, because a `LineCheck` database entry is something completely different thing than a `LineChecker` that checks all the lines.

Reads best commit by commit...
```ruby
:errors_found => incorrect_accounts.any?,
```