Urgh, mirroring nomenclature.
I tend to use:
Primary & Secondary - Server and FailoverPartner respectively (or, Primary = the server you expect to be the 'live' one most of the time).
These titles always refer to the same server.
Principal & Mirror - refer to whichever server is carrying out that role at the time.
So, the errors are on the Primary Server when it is carrying out the Mirror role.