[C#] Improve error messages for views

[rekhoff]

As mentioned in the comments of #4392, error messages can be improved in for Views.
Example error message revisions:

• Move the diagnostic location to the return type (IEnumerable<T>) so the error highlights the offending return.
• C# should use List / T? (nullable) terminology so it matches the actual C# surface API (no Vec/Option in docs).