[```firestartr-bootstrap```] Add validation for default githubgroup and improve the error messages

[frmadem]

Motivation

The current shape of the bootstrap machinery needs a default group that must exist at the github level.

Therefore, we should introduce a validation to ensure that group exists before continuing with the procses.

In addition there should be better messages when the group does not exist.

Acceptance criteria

• A new validation has been added according to the specs.
• The messages have been improved.
• Tests have been made and passed.
• Team acceptance.