Add `MALFORMED_ENTRY_OWNER` error for CODEOWNERS

What does this MR do and why?

Problem

We currently use Entries with spaces for some errors that are actually Malformed owners. We need to change this so we convey a clear message of what's actually happening in the error.

Solution

We should add a new error type MALFORMED_ENTRY_OWNER: Malformed Owners to be more specific about the error we are returning

References

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Navigate to existing or create new CODEOWNERS file
2. Add your CODEOWNERS entries or use this

# Regular owner
*.rb @valid-user

# Just invalid owners
*.js not_a_user_not_an_email

# mixed of valid and invalid
*.js malformed @valid-user @valid-group another_malformed

3. Go to ee/lib/gitlab/code_owners/file.rb and change add_error(Error::INVALID_ENTRY_OWNER_FORMAT, line_number) to add_error(Error::MALFORMED_ENTRY_OWNER, line_number)
4. Refresh CODEOWNERS in gdk
5. You should see the error for line 5.

Related to #517961 (closed)