T7915: Use alert exn attribute for audit of uncaught exceptions at compile time

[jestabro]

Change summary

This adds explicit compiler alerts to track exceptions within vyos1x-config and its use in vyconf and libvyosconfig.
The benefit of this "poor-man's" static analysis is a careful audit of all raised exceptions in any use of the library.
The downside is mildly unpleasant annotations and comments within the code, which will remain temporarily to provide (1) compiler warnings for any additions in vyos1x-config/vycon/libvyosconfig (2) comments describing the exception handling. These will eventually be subsumed by consistent handling using the above as a guide.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe): compiler driven exception audit

Related Task(s)

Related PR(s)

vyos/vyconf#33
vyos/vyos-1x#4814

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[jestabro]

Added minor change to value_exists before exposing in libvyosconfig.

[dmbaturin]

I'm all for more compile-time checks, so let's try this.