fix for 'missing' error with validate_all or validate_always

[samuelcolvin]

Change Summary

Fix model parsing behaviour when validate_all=True or validate_always=True to raise the correct error.

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• HISTORY.rst has been updated
  • if this is the first change since a release, please add a new section
  • include the issue number or this pull request number #<number>
  • include your github username @<whomever>

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@9ef4018). Click here to learn what that means.
The diff coverage is 100%.

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