✨ RawError, RawActivity simplified.

[jortel]

This PR simplifies the API for use cases for which the new RawError() and RawActivity() were meant to support.

Especially the RawActivity(). The main case here is to report multi-line activity. In this case the activity is reporting something followed by multiple lines. The first line is the-activity followed by multiple-lines of detail. This PR simplifies this more than the RawActivity() because it can be achieved in (1) addon.Activity() call.

For example:

addon.Activity("Doing something with a list of things:\n%s", "One\nTwo\nThree")

Results in:

- Doing something with a list of things:
- > One
- > Two
- > Three

The Error and RawError is achieved by reversing which is the edge case. Errorf() is added (instead of RawError) which makes for a more intuitive API.

[mansam]

LGTM