bridge: T4673: raise UnconfiguredSubsystem on non-existent bridge intf

[jestabro]

Change Summary

In the case of non-existent bridge interface, the command, called by _get_raw_data_fdb:

sudo bridge --json fdb show br {bridge}

will return exit code -1. The source code for iproute2 'show_fdb' reveals that this is the only condition to return -1 for that command, consequently we can unambiguously raise the op-mode error 'UnconfiguredSubsystem' in this case.

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):

Related Task(s)

• https://phabricator.vyos.net/Txxxx

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