Fix silent content import failures when author validation fails

[jameslcowan]

Fixes #26268

When a JSON import hits validation errors (bad email, bio too long, etc.), the data importer was throwing a raw array instead of a proper error. Node logs that as rejected with a non-error: [object Array], and the failure email just says to contact support — no hint which author or field failed.

This wraps those errors in a DataImportError with readable messages (including the user email when we have it from import context) and shows those details in the import failure email.

Test plan

• pnpm test in ghost/core (or at least import-errors.test.js)
• Import a JSON export with an invalid users[].email — expect a clear error, not [object Array]
• Import with a bio over 250 chars — same