Add error, with link, for passing Quantity to unit slot

[chiphogg]

Imagine trying to round some size to the nearest 10 bytes:

auto size = bytes(1234);
size = round_as<int>(bytes * mag<10>(), size);

This works (it gives bytes(1230)). However, users might try writing something like this:

auto size = bytes(1234);
size = round_as<int>(bytes(10), size); // (Incorrect)

If so, they might get an error like this:

That's not the best. It does say that IsUnit<...Quantity...> isn't satisfied, but it's not as straightforward as it could be.

We could instead turn AssociatedUnit<Quantity<...>> into a hard compiler error, with a readable message. Something like this:

That seems better: we can directly tell them that they're passing a Quantity to a unit slot, and link them to documentation on unit slots. Actually, probably even better would be to make a new entry in the troubleshooting page.