```
      raise Error::MissingAttributes.new("Missing source_branch.") unless params.has_key?(:source_branch)
      raise Error::MissingAttributes.new("Missing target_branch.") unless params.has_key?(:target_branch)
      raise Error::MissingAttributes.new("Missing title.")         unless params.has_key?(:title)
```

What you are looking for?

With the following in `error.rb` ?

```
    # Raise when attributes are missing.
    class MissingAttributes < Error; end
```