Add `MALFORMED_ENTRY_OWNER` error type for CODEOWNERS

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.

Proposal

We should add a new error type MALFORMED_ENTRY_OWNER: Malformed Owners to be more specific about the error we are returning. This was meant to be added in the syntax MR but never made it in. I found this note on the MR that brought it up but there was no clear resolution.