Unique exit codes

Implemented Uniquie exit codes for the following exits:

Exit code	Reason	Solution
10	Call of 'archspec cpu' failed	Please install archspec, for example via 'pipx install archspec'
20	Requested package set is not available	Use 'available' command to see list of available package_sets
30	Current directory is already initialised	Check if you are in the right directory
40	Current directory is not initialised	Check if you are in the right directory, if so use 'init' command to initialise

Skipped the changes to 1) remove command as its not yet implemented 2) sanity check for mpsd_release as its removed in one of the MR.

Closes #29 (closed)