[config] Use 'click.fail()' in place of 'raise click.Abort' in order …

…to print error messages to user; Other minor tweaks (sonic-net#341)

- Replace calls to 'click.Abort()' with the Click context method 'fail()' which takes an error message as a parameter in order to ensure descriptive messages are printed rather than simply the message, "Aborted"

- Change entrypoint name from 'cli' to 'config'

- Rename a couple helper functions for clarity

- Replace 'print' calls with 'click.echo()' for consistency

- Enhance comments