parser: return the correct error on failure (LP#2000324)

[daniloegea]

If the parsing process finishes with an error, we should return this error instead of assuming it's a missing interface.

For instance, the configuration below will fail with "br0: interface 'ens3' is not defined" when the actual error is "invalid IP family '-1'" due to the invalid IP used as the default route:

network:
  bridges:
    br0:
      interfaces:
        - ens3
      routes:
        - to: 0/0
          via: 100.64.0.1
  ethernets:
    ens3: {}
  version: 2

Description

This PR addresses LP#2000324

The configuration above was failing with:

$ netplan generate --root-dir /workspace/netplan-issues/fakeroot
/workspace/netplan-issues/fakeroot/etc/netplan/static.yaml:5:11: Error in network definition: br0: interface 'ens3' is not defined
        - ens3
          ^

With this change this is the result:

$ LD_LIBRARY_PATH=build/src/ PYTHONPATH=. src/netplan.script generate --root-dir /workspace/netplan-issues/fakeroot
/workspace/netplan-issues/fakeroot/etc/netplan/static.yaml:7:15: Error in network definition: invalid IP family '-1'
        - to: 0/0
              ^

Checklist

• Runs make check successfully.
• Retains 100% code coverage (make check-coverage).
• New/changed keys in YAML format are documented.
• (Optional) Adds example YAML for new feature.
• (Optional) Closes an open bug in Launchpad.

[slyon]

Thanks, lgtm. I've added some small bits to cleanup the npp->missing_id hashmap in the error case. WDYT?