Unclear error message when attempting to add a collection to a work

[elrayle]

To reproduce...

col = Hydra::Works::Collection.new(id: 'col-1')
wrk = Hydra::Works::Work.new(id: 'w-1')

wrk.members << col
=> ActiveFedora::AssociationTypeMismatch: #<Collection:0x007fc234ef9950> is not a PCDM object.

Although a work can have works and file sets as members and both are PCDM objects, it is not the case that all PCDM objects are works and file sets. It would be more precise for this message to read...

wrk.members << col
=> ActiveFedora::AssociationTypeMismatch: #<Collection:0x007fc234ef9950> is not a Hydra::Works work or fileset.

[mjgiarlo]

I'll take a look at this soonish.