Correct error handling of duplicate position code

[gjvoosten]

Show a proper error message when someone tries to create a position with a duplicate code.

Closes AB#813

User changes

• none

Superuser changes

• When a superuser tries to create a position with a code that already exists, a proper error message is shown.

Admin changes

• none

System admin changes

• anet.yml or anet-dictionary.yml needs change
• db needs migration
• documentation has changed
• graphql schema has changed

Checklist

• Described the user behavior in PR body
• Referenced/updated all related issues
• commits follow a repo#issue: Title title format and these 7 rules
• commits have a clean history, otherwise PR may be squash-merged
• Added and/or updated unit tests
• Added and/or updated e2e tests
• Added and/or updated data migrations
• Updated documentation
• Resolved all build errors and warnings
• Opened debt issues for anything not resolved here