configtree: T4961: improve error reporting of function copy

[jestabro]

Change Summary

The function copy in vyos.configtree does not recursively create intermediary nodes of the target path; only the last node of the path is created. However, the error was not being caught/reported in case of a non-existent node. Corrected error reporting is provided in the PR's:
vyos/vyos1x-config#12 (merged)
vyos/libvyosconfig#6 (merged)

and is passed to ConfigTreeError in this PR.

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/T4961

Component(s) name

Proposed changes

How to test

Checklist:

• 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