DLSV2-541 Fixes error and adds validation to add contributor

[kevwhitt-hee]

JIRA link

DLSV2-541

Description

Fixes a bug with the Frameworks add contributor function. The data service method for retrieving info about the chosen administrator referred to a column (FrameworkCollaborators.Active) that wasn't present.

This method also relies on the email address matching a registered admin user but failed quietly if there wasn't one. This change introduces a validator error message to the page explaining why they weren't added.

Screenshots

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Developer checks

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I have:

• Run the formatter and made sure there are no IDE errors.
• Written tests for the changes (accessibility tests, unit tests for controller, data services, services, view models, etc)
• Manually tested my work with and without JavaScript. Full manual testing guidelines can be found here: https://softwiretech.atlassian.net/wiki/spaces/HEE/pages/6703648740/Testing
• Scanned over my own MR to ensure everything is as expected.