Treat 501 errors as neither pass nor fail

[kjetilk]

Following CommunitySolidServer/CommunitySolidServer#798 I think it makes sense to treat 501 as neither pass nor fail, as it is the server explicitly acknowledging that it knows it hasn't implemented a feature.

Earl doesn't have the exact right predicate for this, as the test has been done, and it applies, it just doesn't count towards the server pass or failure counts.

[edwardsph]

We have limited outcomes in the EARL vocab so may need to use earl:failed but add rdfs:comment to highlight the distinction. When creating the report assertions we need to check the status or responseStatus for 501